A New and Improved Population-Based Canadian Reference for Birth Weight for Gestational Age Michael S. Kramer, MD*‡; Robert W. Platt, PhD*‡; Shi Wu Wen, MB, PhD§; K.S. Joseph, MD, PhD\; Alexander Allen, MD\; Michal Abrahamowicz, PhD‡; Béatrice Blondel, PhD¶; and Gérard Bréart, MD, PhD¶ for the Fetal/Infant Health Study Group of the Canadian Perinatal Surveillance System ABSTRACT. Background. Existing fetal growth references all suffer from 1 or more major methodologic problems, including errors in reported gestational age, biologically implausible birth weight for gestational age, insufficient sample sizes at low gestational age, singlehospital or other non-population–based samples, and inadequate statistical modeling techniques. Methods. We used the newly developed Canadian national linked file of singleton births and infant deaths for births between 1994 and 1996, for which gestational age is largely based on early ultrasound estimates. Assuming a normal distribution for birth weight at each gestational age, we used the expectation-maximization algorithm to exclude infants with gestational ages that were more consistent with 40-week births than with the observed gestational age. Distributions of birth weight at the corrected gestational ages were then statistically smoothed. Results. The resulting male and female curves provide smooth and biologically plausible means, standard deviations, and percentile cutoffs for defining small- and large-for-gestational-age births. Large-for-gestational age cutoffs (90th percentile) at low gestational ages are considerably lower than those of existing references, whereas small-for-gestational-age cutoffs (10th percentile) postterm are higher. For example, compared with the current World Health Organization reference from California (Williams et al, 1982) and a recently proposed US national reference (Alexander et al, 1996), the 90th percentiles for singleton males at 30 weeks are 1837 versus 2159 and 2710 g. The corresponding 10th percentiles at 42 weeks are 3233 versus 3086 and 2998 g. Conclusions. This new sex-specific, population-based reference should improve clinical assessment of growth in individual newborns, population-based surveillance of geographic and temporal trends in birth weight for gestational age, and evaluation of clinical or public health interventions to enhance fetal growth. Pediatrics 2001;108(2). URL: http://www.pediatrics.org/cgi/content/ From the Departments of *Pediatrics and of ‡Epidemiology and Biostatistics, McGill University Faculty of Medicine, Montreal, Canada; §Bureau of Reproductive and Child Health, Health Canada, Ottawa, Canada; \Department of Pediatrics, Dalhousie University Faculty of Medicine, Halifax, Nova Scotia, Canada; and the ¶Epidemiological Research Unit on Perinatal and Women’s Health (INSERM), Villejuif, France. Drs Kramer, Platt, and Abrahamowicz are career scientists of the Canadian Institutes of Health Research. Received for publication Aug 21, 2000; accepted Apr 9, 2001. Address correspondence to Michael S. Kramer, MD, 1020 Pine Ave W, Montreal, Quebec, Canada H3A 1A2. E-mail: michael.kramer@mcgill.ca PEDIATRICS (ISSN 0031 4005). Copyright © 2001 by the American Academy of Pediatrics. full/108/2/e35; fetal growth, birth weight, gestational age, preterm birth, postterm birth. ABBREVIATIONS. SD, standard deviation; LGA, large for gestational age; SGA, small for gestational age. F or more than half a century, clinicians and investigators have proposed reference data for assessing birth weight for gestational age. These references have been used by clinicians and researchers to assess fetal growth in individual infants and in populations.1 Unfortunately, none of the available references are entirely satisfactory. Some are hospital-based,2–7 giving rise to potential selection bias and problems of generalizability, particularly in view of the low2,6,7 or high4,5 socioeconomic status or high altitude2 that characterizes some of the study hospitals; others are prescriptive rather than descriptive, ie, they are based on infants without known risk factors for impaired fetal growth and thus may not be applicable to populations with mixtures of low- and high-risk pregnancies.8,9 Some are unisex references that fail to account for the known larger birth weight for gestational age in male versus female infants3,4,6,10; others11,12 go to the opposite extreme and provide curves that are specific for different races, parity, maternal size, and other customizing features for which available data do not permit confident inferences as to whether variations in fetal growth are physiologic or pathologic. Finally, some references are now several decades old and may no longer be pertinent to infants born in more recent years, given the increase in the size of infants born at or near term over the last several decades. The largest problem with existing references for birth weight for gestational age, however, relates to the measurement of gestational age. Early references rounded gestational age to the nearest week,2– 6 rather than truncated to completed weeks. Although this practice makes sense from a biological standpoint, it is not consistent with World Health Organization (WHO; International Classification of Diseases) recommendations to base gestational age on the number of completed weeks, and references based on the nearest week cannot be applied to populations complying with the WHO recommendations. But even when measured in completed weeks, gestational ages in older references are flawed by http://www.pediatrics.org/cgi/content/full/108/2/e35 PEDIATRICS Vol. 108 No. 2 August 2001 Downloaded from by guest on October 24, 2016 1 of 7 being based on the date of onset of the last menstrual period,2– 6,10,13–16 which has been shown to grossly overestimate the proportion of infants with postterm gestational ages and underestimate those born preterm when compared with early ultrasound measurements.17 This leads to artifactually wide, or even bimodal, distributions of birth weight at very low gestational ages, and flattening or declining curves postterm, despite the use of various statistical or clinical methods for excluding biologically implausible combinations of birth weight and gestational age.10,14,16,18,19 Finally, sample sizes for many references are too small, particularly at low gestational ages, leading to irregularity and even nonmonotonicity in the shape of fetal growth curves at these low gestational ages. Some recent references have not smoothed the curves to correct these irregularities.20,21 In this study, we have attempted to overcome these previous deficits by constructing a sex-specific reference of birth weight for gestational age that has several advantages over existing references. It is based on a linked live birth-infant death file for all infants born in all of the Canadian provinces and territories (with the exception of Ontario) born between 1994 and 1996, during which time gestational ages have reflected the increasing use of early ultrasound for gestational age estimation. Gestational age is measured in completed weeks, and we have used a new mixture distribution method to exclude infants with implausible gestational ages. This results in smooth, monotonic curves with biologically sensible distributions at all gestational ages. We present both a graphical depiction and a tabular representation of means, standard deviations (SDs), and the 3rd, 5th, 10th, 50th (median), 90th, 95th, and 97th percentiles in the hope that they will be useful to clinicians in classifying fetal growth of newborn infants under their care and of stillbirths, and to researchers and public policy makers in comparing geographic differences and temporal trends in birth weight for gestational age in populations. METHODS The references we constructed were based on all births contained in the linked file of live births and infant deaths occurring in the provinces and territories of Canada (with the exception of Ontario) born between January 1, 1994, and December 31, 1996. (The linked file was used, rather than the entire Canadian Birth Database, because the linkage procedure allowed elimination of several duplicates contained in the Birth Database.) Ontario was excluded from the study population base for constructing these references because of documented problems with data quality.22 Canadian birth certificates include gestational age recorded in completed weeks. Although no information is contained on the birth certificate about the method used to assess gestational age, nor documentation of the dates and results of ultrasound estimates, careful evaluation of these data over several decades strongly suggests that early ultrasound has increasingly been the basis for gestational age assessments in recent years.23,24 No information on racial origin is contained on Canadian birth certificates. Our proposed reference is based on singletons with recorded gestational ages of 22 to 43 weeks. The reference sample comprises 347 570 males and 329 035 females. We used a mixture distribution method for correcting gestational age errors, a modification of the procedure described by Oja et al.25 The model of Oja et al assumes a log normal distribution of birth weight at each gestational age 2 of 7 and errors of 14 weeks or 24 weeks. As assumed in previous studies12,16 and as confirmed by ultrasound-based estimated fetal weights,26 our method assumes that the true distribution of birth weight at each gestational age is normal (Gaussian). We also assume that the observed gestational ages are a mixture of correct values and of term (40-week) gestations erroneously measured at the observed (recorded) gestational age. This latter assumption is consistent with the frequently observed curve “bump” at the upper percentiles for extremely preterm infants.10,14,16 We used the estimation-maximization algorithm27 to derive maximum likelihood estimates of the mean and SD and of the probability that the true gestational age of an individual infant was the observed (recorded) gestational age rather than 40 weeks. Starting values for the mean and SD parameters were estimated from the lower half of the observed distribution of birth weight for gestational age (which should not be affected by misclassifying infants born at term as extremely preterm). We then resampled all observations. This resampling resulted in either maintaining the infant in the dataset with the recorded gestational age or deleting that infant, with the probability of deletion equal to the estimated probability that the true gestational age was 40 weeks. Finally, the percentiles of the birth weight distribution at the corrected gestational ages were generated using a smoothing spline with 7 degrees of freedom,28 weighted by the square root of the (corrected) number of infants at each gestational age. Tables and graphs were created separately for males and females for the 3rd, 5th, 10th, 50th (median), 90th, 95th, and 97th percentiles at 22 to 43 completed weeks, based on the smoothed estimated curves, and for the mean and SD calculated from the empirical distribution of birth weights after correction. The means and SDs are also tabulated to allow calculation of z scores rather than (or in addition to) percentiles, where z5 observed birth weight 2 mean birth weight SD RESULTS Tables 1 and 2 show the 3rd, 5th, 10th, 50th (median), 90th, 95th, and 97th percentile; mean; and SD birth weights for male and female infants, respectively. Figures 1 and 2 show the crude percentile graphs, and Figs 3 and 4 show the curves produced after correcting the gestational ages and smoothing the resulting birth weight distributions. The crude curves show a convex “bump” in the upper percentiles at 27 to 33 weeks, consistent with our model’s assumption that some truly term births are misclassified at these (grossly underestimated) gestational ages. The corrected curves are smooth and evenly spaced and show no biologically implausible bumps preterm or flattening or declines postterm. Moreover, each of the percentile curves shows the expected sigmoid shape, with birth weight increasing monotonically as gestational age advances. The smoothed curves closely follow (within limits of random error) the empiric birth weight distribution at each (corrected) gestational age. For example, at 30 weeks, the 90th percentile curve cuts off 91.4% of males and 87.7% of females in our data set. At 42 weeks, the 10th percentile curve cuts off 10.4% of males and 10.2% of females. Figure 5 illustrates how our procedure has corrected the crude curves at 30 weeks’ gestation (for males). At low percentiles, the curves before and after correction are virtually identical, but starting at the 90th percentile, and especially at the 95th and 97th percentiles, the cutoffs after the correction are substantially lower. Table 3 compares our new proposed reference (based on the corrected curves) with 4 population- POPULATION-BASED CANADIAN REFERENCE FOR BIRTH WEIGHT Downloaded from by guest on October 24, 2016 TABLE 1. Gestational Age 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 Birth Weight (g) for Gestational Age, Canadian Male Singletons Born Between 1994 and 1996, Corrected and Smoothed n* 82 114 156 202 234 254 330 392 467 584 997 1368 2553 4314 9648 19 965 51 947 77 623 112 737 54 139 8791 276 3rd Percentile 5th Percentile 10th Percentile 50th Percentile 90th Percentile 95th Percentile 97th Percentile Mean SD 338 406 468 521 571 627 694 780 885 1012 1164 1344 1552 1783 2024 2270 2498 2684 2829 2926 2960 2954 368 434 498 557 614 677 752 845 959 1098 1266 1460 1677 1907 2144 2384 2605 2786 2927 3025 3070 3081 401 475 547 617 686 763 853 964 1099 1259 1444 1648 1866 2091 2321 2552 2766 2942 3079 3179 3233 3249 490 589 690 795 908 1033 1173 1332 1507 1698 1906 2127 2360 2600 2845 3080 3290 3465 3613 3733 3815 3864 587 714 844 981 1125 1278 1445 1629 1837 2069 2319 2580 2851 3132 3411 3665 3877 4049 4200 4328 4433 4528 627 762 902 1048 1200 1358 1532 1729 1955 2209 2478 2750 3029 3318 3604 3857 4065 4232 4382 4512 4631 4747 659 797 940 1092 1251 1416 1598 1809 2053 2327 2614 2897 3184 3475 3759 4003 4202 4361 4501 4631 4773 4941 501 598 697 800 909 1026 1159 1312 1487 1682 1896 2123 2361 2607 2855 3091 3306 3489 3638 3745 3800 3793 111 114 125 147 178 209 241 273 306 339 369 391 410 428 443 449 448 445 447 459 485 527 * Sample size at each gestational age after exclusions. TABLE 2. Gestational Age 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 Birth Weight (g) for Gestational Age, Canadian Female Singletons Born Between 1994 and 1996, Corrected and Smoothed n* 80 106 148 184 191 188 287 299 390 461 795 1055 2018 3391 8203 17 308 47 516 75 068 110 738 52 063 7970 277 3rd Percentile 5th Percentile 10th Percentile 50th Percentile 90th Percentile 95th Percentile 97th Percentile Mean SD 332 379 424 469 516 569 634 716 814 938 1089 1264 1467 1695 1935 2177 2406 2589 2722 2809 2849 2862 347 403 456 508 562 624 697 787 894 1026 1184 1369 1581 1813 2052 2286 2502 2680 2814 2906 2954 2975 385 450 513 578 645 717 802 903 1022 1168 1346 1548 1768 1998 2227 2452 2658 2825 2955 3051 3114 3159 466 557 651 751 858 976 1109 1259 1427 1613 1817 2035 2266 2506 2744 2968 3169 3334 3470 3576 3655 3717 552 669 790 918 1060 1218 1390 1578 1783 2004 2242 2494 2761 3037 3307 3543 3738 3895 4034 4154 4251 4333 576 706 839 982 1139 1313 1499 1701 1918 2150 2399 2664 2948 3242 3523 3752 3931 4076 4212 4330 4423 4495 576 726 887 1060 1247 1446 1657 1885 2121 2347 2578 2825 3097 3384 3660 3886 4061 4202 4331 4444 4554 4685 472 564 656 754 860 976 1107 1256 1422 1604 1808 2029 2266 2512 2754 2981 3181 3350 3486 3588 3656 3693 72 95 121 152 186 222 254 286 319 345 368 389 409 426 439 443 439 434 434 439 448 459 * Sample size at each gestational age after exclusions. based references: 1 from Canada (Arbuckle et al20) and 3 from the United States (Williams et al16 [white non-Hispanics only], Zhang and Bowes12 [whites only], and Alexander et al19). Tabulated values for the Canadian reference were provided by Statistics Canada (Russell Wilkins, Health Analysis and Modeling Group, Statistics Canada, Ottawa, Ontario, personal communication). The comparison focuses on the 2 sensitive regions mentioned earlier: the 90th percentile (the conventional large-for-gestational-age [LGA] cutoff) at 30 weeks, and the 10th percentile (the conventional small-for-gestational-age [SGA] cutoff) at 42 weeks. Each of the previous references uses a correction procedure to reduce errors in pre- term gestational age estimates, but each is characterized by residual bumps preterm and/or flattening or declining curves postterm. Our curves yield the lowest LGA cutoff at 30 weeks, with differences as large as 900 g. Our curves also yield the highest SGA cutoff at 42 weeks, with differences up to 400 g. All 4 of our cutoffs, however, are close to those of the earlier Canadian reference by Arbuckle et al.20 The mean values also differ at these gestational ages. Our reference mean for males is 1487 g at 30 weeks and 3800 g at 42 weeks. The corresponding figures for Williams et al16 are 1537 and 3665 g; for Zhang and Bowes,12 1653 and 3548 g; and for Alexander et al,19 1637 and 3522 g. http://www.pediatrics.org/cgi/content/full/108/2/e35 Downloaded from by guest on October 24, 2016 3 of 7 Fig 1. Canadian crude curves. male singletons, Fig 2. Canadian crude curves. DISCUSSION The reference curves and tables presented herein differ in several important respects from those currently available. Their main advantages include their recent vintage; population base; increased reliance on early ultrasound-based estimates of gestational age; statistical modeling of gestational ages to correct biologically implausible values (based on the observed and expected birth weights); sex-specificity; and adaptability to the use of either percentiles or z scores. This reference differs from others currently available in showing lower means and particularly upper (90th, 95th, and 97th) percentile birth weights at low gestational ages, as well as higher birth weights (particularly at the 10th, 5th, and 3rd percentiles) for gestational age in the postterm period. 4 of 7 female singletons, The absence of a downturn in the curves in the postterm period is consistent with evidence based on early ultrasound-based gestational ages.29,30 Our reference also differs from older references by reflecting the recent temporal trend toward increasing birth weight at or near term. This new reference should prove helpful for several types of users in developed country settings. For clinicians, it enables better classification of individual infants as small, appropriate, or large for their gestational age. It should prove useful for clinicians caring for extremely preterm infants in whom unexpectedly high birth weights should lead to skepticism about the reported gestational age. It should also be reassuring to prenatal care providers by eliminating the impression that fetuses fail to gain, or POPULATION-BASED CANADIAN REFERENCE FOR BIRTH WEIGHT Downloaded from by guest on October 24, 2016 Fig 3. Canadian male singletons, corrected and smoothed curves. Fig 4. Canadian female singletons, corrected and smoothed curves. even lose, weight postterm. For researchers, this new reference should allow better classification of groups of infants for determination of geographic differences, temporal trends, etiologic determinants, and short- and long-term prognosis. For public health policy makers, the reference should be useful in surveillance, ie, in tracking population differences by geographic location, socioeconomic status, race/ ethnicity, and other relevant factors, as well as in tracking trends over time and in response to educational or other public health interventions. Despite the advantages of our new reference over its predecessors, several limitations should be acknowledged. First, like any purely statistical correction procedure, our method compares poorly with early ultrasound for valid estimation of the true gestational age.17 In particular, early ultrasound corrects last menstrual period-based estimates by a systematic shift to the left (ie, toward lower gestational ages) across the entire gestational age distribution, primarily because of the effect of delayed ovulation.31 Our method, however, corrects only gross errors by excluding preterm (and some postterm) infants whose birth weights are more compatible with 40-week births. We have also experimented with more complex statistical corrections (eg, 61, 2, 4, and 8 weeks), but the more complex algorithms are far more computer-intensive, result in many more exclusions, and do not substantially alter the slopes or SGA or LGA cutoffs of the reference curves. The most important limitation of all populationbased references, including ours, is their cross-sectional nature, ie, they are based on the birth weights of different infants born at different gestational ages, http://www.pediatrics.org/cgi/content/full/108/2/e35 Downloaded from by guest on October 24, 2016 5 of 7 Fig 5. Birth weight distribution at 30 weeks, male singletons, before and after correction. TABLE 3. Comparison of Key Birth Weight Cutoffs (g), Proposed New Reference Versus Four Previous References Reference Proposed reference Arbuckle et al20 Williams et al16 Zhang and Bowes12 Alexander et al19 LGA (90th Percentile) at 30 Weeks SGA (10th Percentile) at 42 Weeks Male Female Male Female 1837 1870 2159 2355* 2710† 1783 1800 2113 2288* 2710† 3233 3200 3086 2835* 2998 3114 3070 2936 2735* 2884 * Average of values for primiparas and multiparas. † Sex-specific value not reported. rather than longitudinal measurements of the same infants over the course of gestation.32 Growth is defined as an increase in size over time, and documentation of increasing size therefore requires 2 or more serial measurements. Unfortunately, serial anthropometric measurements during fetal life are feasible only with ultrasound, and these have not proved to be sufficiently valid or precise to serve as a reference.33–35 The substitution of cross-sectional for longitudinal data on fetal growth is a problem, because evidence suggests that preterm infants are somewhat smaller than fetuses of the same gestational age who remain in utero.36 –39 This problem may partly reflect the fact that some of the determinants of fetal growth and gestational duration overlap. Pregnancyinduced hypertension and preeclampsia, for example, are risk factors for both intrauterine growth restriction and both spontaneous and induced preterm delivery.39 – 42 At or after term, fetuses who remain unborn may not have grown at the same rate as those born earlier; because fetal size is considered to be one of the determinants of the onset of labor, cross-sectional data may reflect the earlier birth of fastergrowing fetuses.1 If and when ultrasound technology improves to permit more valid and reliable measurements of fetal weight and other anthropometric measurements in utero, a truly longitudinal fetal growth reference will be feasible. Other future changes may also require modification or replacement of our reference. Temporal trends toward increasing maternal prepregnancy weight and weight gain during pregnancy, maternal stature, and reductions in cigarette smok6 of 7 ing will probably continue to increase the size of infants born at or near term. Finally, the increasing trend toward obstetric intervention to hasten delivery for pathologic pregnancies in the preterm period, and in response to signs of slow growth or other fetal compromise in the postterm period, may continue to affect the shape of the growth curve at these periods of gestation. REFERENCES 1. World Health Organization Expert Committee on the Use and Interpretation of Anthropometry. Physical Status: The Use and Interpretation of Anthropometry. Geneva, Switzerland: World Health Organization; 1995 2. Lubchenco L, Hansman C, Dressler M, Boyd E. Intrauterine growth as estimated from liveborn birth weight data at 24 to 42 weeks of gestation. Pediatrics. 1963;32:793– 800 3. Gruenwald P. Growth of the human fetus. I. Normal growth and its variation. Am J Obstet Gynecol. 1966;94:1112–1119 4. Usher R, McLean F. Intrauterine growth of live-born Caucasian infants at sea-level: standards obtained from measurements in 7 dimensions of infants born between 25 and 44 weeks gestation. J Pediatr. 1969;74: 901–910 5. Babson S, Behrman R, Lessel R. Liveborn birth weights for gestational age of white middle class infants. Pediatrics. 1970;45:937–943 6. Brenner W, Edelman D, Hendricks C. A standard of fetal growth for the United States of America. Am J Obstet Gynecol. 1976;126:555–564 7. Amini S, Catalano P, Hirsch V, Mann L. An analysis of birth weight by gestational age using a computerized perinatal data base, 1975–1992. Obstet Gynecol. 1994;83:342–352 8. Lawrence C, Fryer J, Karlberg J, Niklasson A, Ericson A. Modelling of reference values for size at birth. Acta Paediatr Scand. 1989;350(suppl): 55– 69 9. Niklasson A, Ericson A, Fryer J, Karlberg J, Lawrence C, Karlberg P. An update of the Swedish reference standards for weight, length, and head circumference at birth for given gestational age (1977–1981). Acta Paediatr Scand. 1991;89:756 –762 10. David R. Population-based intrauterine growth curves from computerized birth certificates. South Med J. 1983;76:1401–1406 11. Gardosi J, Chang A, Kalyan B, Sahota D, Symonds E. Customized antenatal growth charts. Lancet. 1992;339:283–287 12. Zhang J, Bowes W Jr. Birth-weight-for-gestational-age patterns by race, sex, and parity in the United States population. Obstet Gynecol. 1995;86: 200 –208 13. Gibson J, McKeown T. Observations of all births (23,970) in Birmingham, 1947;VI:Birth weight, duration of gestation, and survival related to sex. Br J Soc Med. 1952;6:152–158 14. Battaglia F, Frazier T, Hellegers A. Birth weight, gestational age, and pregnancy outcome, with special reference to high birth weight-low gestational age infant. Pediatrics. 1966;37:417– 422 15. Thomson A, Billewicz W, Hytten F. The assessment of fetal growth. J Obstet Gynaecol Br Common. 1968;75:903–916 16. Williams R, Creasy R, Cunningham G, Hawes W, Norris F, Tashiro M. Fetal growth and perinatal viability in California. Obstet Gynecol. 1982; 59:624 – 632 17. Kramer M, McLean F, Boyd M, Usher R. The validity of gestational age POPULATION-BASED CANADIAN REFERENCE FOR BIRTH WEIGHT Downloaded from by guest on October 24, 2016 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. estimation by menstrual dating in term, preterm, postterm gestations. JAMA. 1988;260:3306 –3308 Källén B. A birth weight for gestational age standard based on data in the Swedish Medical Birth Registry, 1985–1989. Eur J Epidemiol. 1995;11: 601– 606 Alexander G, Himes J, Kaufman R, Mor J, Kogan M. A United States national reference for fetal growth. Obstet Gynecol. 1996;87:163–168 Arbuckle T, Wilkins R, Sherman G. Birth weight percentiles by gestational age in Canada. Obstet Gynecol. 1993;81:39 – 48 Roberts C, Mueller L, Hadler J. Birth weight percentiles by gestational age, Connecticut 1988 –1993. Conn Med. 1996;60:131–140 Joseph K, Kramer M. Recent trends in infant mortality rates and proportions of low-birth-weight live births in Canada. Can Med Assoc J. 1997;157:535–541 Arbuckle T, Sherman G. An analysis of birth weight by gestational age in Canada. Can Med Assoc J. 1989;140:157–165 Joseph K, Kramer M. Recent versus historical trends in preterm birth in Canada. Can Med Assoc J. 1999;161:1409 Oja H, Koiranen M, Rantakallio P. Fitting mixture models to birth weight data: a case study. Biometrics. 1991;47:883– 897 Marsál K, Persson P-H, Larsen T, Lilja H, Selbing A, Sultan B. Intrauterine growth curves based on ultrasonically estimated foetal weights. Acta Paediatr. 1996;85:843– 848 Dempster A, Laird N, Rubin D. Maximum likelihood from incomplete data via the EM algorithm. J R Stat Soc, Series B. 1997;39:1–38 Hastie T, Tibshirani R. Generalized Additive Models. New York, NY: Chapman & Hall/CRC; 1990 McLean F, Boyd M, Usher R, Kramer M. Postterm infants: too big or too small? Am J Obstet Gynecol. 1991;164:619 – 624 Wilcox M, Gardosi J, Mongelli M, Ray C, Johnson I. Birth weight from 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. pregnancies dated by ultrasonography in a multicultural British population. Br Med J. 1993;307:588 –591 Yang H, Kramer M, Platt R, et al. How does early ultrasound estimation of gestational age lead to higher rates of preterm birth? Am J Obstet Gynecol. 2001; in press Wilcox A. Birth weight, gestation, and the fetal growth curve. Am J Obstet Gynecol. 1981;139:863– 867 Little G. Fetal growth and development. In: Eden R, Boehm F, eds. Assessment and Care of the Fetus: Physiological, Clinical, and Medico-Legal Principles. Norwalk, CT: Appleton & Lange; 1990:1–5 Rogers M, Chung T, Chang A. Ultrasound fetal weight estimation: precision or guess work? Aust N Z J Obstet Gynaecol. 1993;33:142–144 Chauhan S, Charania S, McLaren R, et al. Ultrasonographic estimate of birth weight at 24 to 34 weeks: a multicenter study. Am J Obstet Gynecol. 1998;179:916 Ott W, Doyle S. Normal ultrasonic fetal weight curve. Obstet Gynecol. 1982;59:603– 606 Weiner C, Sabbagha R, Vaisrub N, Depp R. A hypothetical model suggesting suboptimal intrauterine growth in infants delivered preterm. Obstet Gynecol. 1985;65:323–326 Secher N, Hansen P, Thomsen B, Keiding N. Growth retardation in preterm infants. Br J Obstet Gynaecol. 1987;94:115–120 Hediger M, Scholl T, Schall J, Miller L, Fischer R. Fetal growth and the etiology of preterm delivery. Obstet Gynecol. 1995;85:175–182 Kramer M. Determinants of low birth weight: methodological assessment and meta-analysis. Bull World Health Organ. 1987;65:663–737 Ananth C, Savitz D, Luther E, Bowes W Jr. Preeclampsia and preterm birth subtypes in Nova Scotia, 1986 to 1992. Am J Perinatol. 1997;14:17–23 Kramer M, Séguin L, Lydon J, Goulet L. Socio-economic disparities in pregnancy outcome: why do the poor fare so poorly? Paediatr Perinat Epidemiol. 2000;14:194 –210 http://www.pediatrics.org/cgi/content/full/108/2/e35 Downloaded from by guest on October 24, 2016 7 of 7 A New and Improved Population-Based Canadian Reference for Birth Weight for Gestational Age Michael S. Kramer, Robert W. Platt, Shi Wu Wen, K.S. Joseph, Alexander Allen, Michal Abrahamowicz, Béatrice Blondel, Gérard Bréart and for the Fetal/Infant Health Study Group of the Canadian Perinatal Surveillance System Pediatrics 2001;108;e35 DOI: 10.1542/peds.108.2.e35 Updated Information & Services including high resolution figures, can be found at: /content/108/2/e35.full.html References This article cites 38 articles, 7 of which can be accessed free at: /content/108/2/e35.full.html#ref-list-1 Citations This article has been cited by 29 HighWire-hosted articles: /content/108/2/e35.full.html#related-urls Post-Publication Peer Reviews (P3Rs) One P3R has been posted to this article: /cgi/eletters/108/2/e35 Subspecialty Collections This article, along with others on similar topics, appears in the following collection(s): Fetus/Newborn Infant /cgi/collection/fetus:newborn_infant_sub Permissions & Licensing Information about reproducing this article in parts (figures, tables) or in its entirety can be found online at: /site/misc/Permissions.xhtml Reprints Information about ordering reprints can be found online: /site/misc/reprints.xhtml PEDIATRICS is the official journal of the American Academy of Pediatrics. A monthly publication, it has been published continuously since 1948. PEDIATRICS is owned, published, and trademarked by the American Academy of Pediatrics, 141 Northwest Point Boulevard, Elk Grove Village, Illinois, 60007. Copyright © 2001 by the American Academy of Pediatrics. All rights reserved. Print ISSN: 0031-4005. Online ISSN: 1098-4275. Downloaded from by guest on October 24, 2016 A New and Improved Population-Based Canadian Reference for Birth Weight for Gestational Age Michael S. Kramer, Robert W. Platt, Shi Wu Wen, K.S. Joseph, Alexander Allen, Michal Abrahamowicz, Béatrice Blondel, Gérard Bréart and for the Fetal/Infant Health Study Group of the Canadian Perinatal Surveillance System Pediatrics 2001;108;e35 DOI: 10.1542/peds.108.2.e35 The online version of this article, along with updated information and services, is located on the World Wide Web at: /content/108/2/e35.full.html PEDIATRICS is the official journal of the American Academy of Pediatrics. A monthly publication, it has been published continuously since 1948. PEDIATRICS is owned, published, and trademarked by the American Academy of Pediatrics, 141 Northwest Point Boulevard, Elk Grove Village, Illinois, 60007. Copyright © 2001 by the American Academy of Pediatrics. All rights reserved. Print ISSN: 0031-4005. Online ISSN: 1098-4275. Downloaded from by guest on October 24, 2016