From 1990 to 2005, birth weight decreased among term births in the United States, especially after 1999. These declines were not explained by statistical adjustment for maternal and neonatal characteristics or obstetric practices, such as cesarean delivery, induction of labor, and use of prenatal ultrasonography, and were similar in analyses restricted to a homogenous subset of low-risk mothers. Large for gestational age birth decreased, whereas small for gestational age birth increased.
Strengths of this study include the large nationally representative sample. We had information on a large number of maternal and obstetric characteristics associated with fetal growth and gestational age. Limitations are inherent to birth record data. Birth weight tends to be well recorded, but other factors including gestational length may not be as accurate. The Natality datasets do not include an estimate of gestational length based on ultrasound dating. However, gestational age calculated by the date of the last menstrual period tends to be most accurate among term births.20
The percentage of missing data varied over time for some variables, most notably gestational weight gain (12% missing in 1990 compared with 4% in 2005). However, declines in birth weight were even stronger among the subset of low-risk mothers with no missing data. It is possible that maternal characteristics were not well recorded. It is unlikely that the introduction of the 2003 version of the birth certificate would explain our results, as most factors, including birth weight and gestational age at birth, were recorded identically on the two forms, and downward trends in fetal growth preceded adoption of the revised certificate. Information on augmentation of labor was not available on the 1989 birth certificate, and in some cases induction of labor might be miscoded as augmentation, or vice versa. Maternal prepregnancy BMI, height, and use of assisted reproductive technologies were also not available. As these variables are included on the 2003 birth certificate revision,21
future studies might be better able to track their associations with birth outcomes.
Previous studies, most including only births that occurred before 2000, have evaluated trends in fetal growth and its contributors. In a hospital-based study in Canada using data from 1978 to 1996, trends in maternal characteristics such as increasing prepregnancy BMI and gestational weight gain and decreasing prevalence of smoking during pregnancy explained the observed increases in birth weight and fetal growth.7
Similarly, among neonates born in Sweden from 1992 to 2001, a 23% increase in LGA among term singleton births was explained by concurrent increases in maternal BMI and decreases in smoking.12
Increases in births greater than 4,000 g in Denmark from 1990 to 1999 were explained by changes in maternal prepregnancy weight, height, smoking habits, educational level, and caffeine intake.22
In an analysis of term neonates from 1985 to 1998 using U.S. and Canadian birth data, increases in birth weight and LGA, and decreases in SGA, were attributed to preterm obstetric induction and preterm cesarean delivery.2
A recent study from Sweden reported that, after adjustment for gestational length, age, smoking, parity, and employment, birth weight increased within all maternal BMI categories from 1978 to 1992, but thereafter decreased among neonates born to normal-weight women.23
In the present study, the maternal characteristics routinely recorded on the birth certificate did not appear to be responsible for observed decreases in fetal growth. This observation is concordant with the fact that the directions of trends in all maternal characteristics have continued since the early 1990s without any reversals. Since these trends explained past increases in birth weight and fetal growth, they could not explain recent decreases. Induction of labor also steadily increased each year after 1990. Although rates of cesarean deliveries declined in the early 1990s and subsequently increased after 1997, declines in fetal growth preceded 1997 among some subgroups and were similar in neonates delivered by cesarean and vaginal routes. Thus, it is unlikely that shifts in cesarean delivery practices account for observed trends in fetal growth.
Gestational age is a strong contributor to birth weight. From 1990 to 2005, mean gestational length among U.S. term births decreased by more than 2 days, and the odds of birth on or after the “due date” decreased by more than 40%. Decreases in gestation duration occurred among neonates born by vaginal delivery or by cesarean delivery and among those whose labor was or was not induced. Neither statistical adjustment for maternal and neonatal characteristics and obstetric practices nor restriction of the sample to a homogenous low-risk subgroup–none of whom were induced or delivered by cesarean section–attenuated observed declines in gestational duration among these term births. Similarly, among all subgroups, the proportion of births at 40–41 weeks declined in parallel with mean gestational length (data not shown).
However, the observed decreases in gestational length did not entirely explain the declines in birth weight, fetal growth, and LGA birth, which were somewhat less strong but still persisted after statistical adjustment for gestational age at birth and within specified weeks of gestation. Gestational age at birth is reported on birth certificates in completed weeks and not more finely. It is possible that a decrease of gestational length of a few days within each gestational week might account for the observed declines in fetal growth. If, for example, babies were less likely over time to be born at 39 6/7 weeks and more likely to be born at 39 1/7 weeks, there would be an apparent decline in birth weight at 39 weeks that in fact resulted from a decrease in gestational length rather than a true decrease in fetal growth. However, if such differences in gestational length occurred universally (eg, other neonates who would have been born at 40 1/7 weeks were now born at 39 6/7 weeks), we would observe no such decline in birth weight.
Change over time in the assessment of gestational age may affect observations of trends in gestational length and birth weight for gestational age, although less strongly among term than preterm neonates.24
If smaller neonates were over time increasingly likely to be classified as term rather than preterm, then we would see a downward trend in birth weight for gestational age, especially in gestational weeks 37–38. However, the decline in birth weight was in fact most pronounced in neonates born at 40–41 weeks of gestation. Alternatively, if gestational age at delivery was increasingly over time reclassified lower, that might explain the trends that we observed, as neonates born at 42 weeks or greater tend to be smaller than those born at 40–41 weeks, who are in turn larger than those born at 37–38 weeks.18
However, we also saw decreasing size at birth among the low-risk subgroup, all of whom had first-trimester prenatal care and a prenatal ultrasonography, and presumably equally accurate determination of gestational age over time.
It is also possible that neonates with faster intra-uterine growth were over time increasingly more likely to be delivered earlier because of greater use of cesarean delivery or induction of labor for these larger neonates. In this case, we would expect to see declines in birth weight that were greatest in the later gestational ages, as we did observe. In the regression analysis, however, we did not see evidence that adjustment for obstetric management practices, namely prenatal ultrasonography, induction of labor, and cesarean delivery, attenuated the estimates of decreasing fetal growth over time, but rather seemed to strengthen these estimates. Similarly, birth weight declined even among the homogenous, low-risk subset of mothers, and among all mothers who had neither cesarean delivery nor induction of labor. Classification of cesarean delivery on birth certificates is quite accurate, with 99.8% sensitivity.25
However, induction of labor may not be as accurately recorded, and thus some births that were in fact induced may have been misclassified.
Maternal prepregnancy BMI, a strong contributor to fetal growth,7
was not included on the 1989 birth certificate version. However, since BMI among women of childbearing age has increased over time,26
we would expect that accounting for increasing prepregnancy BMI trends should adjust fetal growth estimates further downward, as we observed with our relatively crude adjustment using mean BMI obtained from the National Health and Examination Survey.
Other factors not recorded in birth records that might contribute to declines in gestational length or fetal growth include trends in maternal diet, physical activity, stress, socioeconomic factors, pollution or toxicant exposures, or prevalence of other, unrecorded medical conditions such as asthma. More detailed studies of smaller populations would be needed to explore the role of these factors.
In this study of term singleton births in the United States from 1990 to 2005, we observed decreases in birth weight that were not explained by maternal and neonatal characteristics or by trends in induced labor or cesarean delivery. Parallel declines in gestational age at birth did not entirely explain the decreasing birth weight. Size at birth predicts not only short-term complications but also long-term health and chronic disease risk even among term births.27
Although the consequences of the modest differences over time in birth weight for gestational age that we observed here are uncertain, any underlying reasons for such a decline may themselves directly influence child health. Therefore, active investigation into the determinants and longer-term sequelae of declining fetal growth among term births is warranted.