Birthweight is increasing in the United States

The rates of obesity and diabetes mellitus have increased over the last several decades.^1,2 This has resulted in a higher number of large-for-gestational-age (LGA) neonates.³ The US cesarean delivery rate has increased over the same period; LGA fetuses may be a contributing factor.⁴ This study aimed to establish whether birthweights in the United States have increased over time.

This was a retrospective cohort study conducted using live birth data of singleton pregnancies from the US National Vital Statistics between January 1972 and December 2020. These data are deidentified and publicly available; therefore, the study was deemed exempt from our institutional review board. Singleton births between 37 0/7 and 42 6/7 weeks of gestation were included. Multiple pregnancies and deliveries which had unknown gestational age or birthweight, were excluded. The mean birthweight by each gestational age week for the years 1972-2020 was calculated at selected years (1972, 1982, 1992, 2002, 2012, 2018, and 2020). Of note, 2018 was included to account for differences in the dataset that might be due to the COVID-19 pandemic. Using R statistical software, a linear model was fit for each gestational age week. A t test was performed to determine whether the slope was statistically different from zero (indicating whether there was a trend of rising or decreasing birthweights over time).

A total of 19,730,588 individuals met the inclusion criteria. However, <1% of the data were excluded because of missing data. Birthweight at 39, 40, 41, and 42 weeks of gestation showed a statistically significant increase over time (Figure). There was a significant decrease in birthweight at 37 weeks of gestation. Data on Hispanic ethnicity became available in 1992. After 1992, birthweight by race or ethnicity group was examined. Each race or ethnicity group echoes the overall trends observed. However, for 39, 40, 41, and 42 weeks of gestation, the non-Hispanic Black and Hispanic groups have higher increases in birthweight than the non-Hispanic White group. There were fluctuations in the overall combined mean for 37 to 42 weeks of gestation (Table).

Although the overall mean birthweight did not increase over the study period, it increased for each gestational age week at ≥39 weeks of gestation. The birthweight at 37 weeks of gestation decreased. The reason for the decrease in birthweight is unclear. Contributing factors may include changes in guidelines on the timing of delivery and method of calculation of gestational age. Increases in the rates of obesity and diabetes mellitus could be contributing to the birthweight increase from 39 weeks of gestation.^1,2 There was an increase in the rate of gestational diabetes mellitus from 3% to 8% in our study population from 1992 to 2020 and an increase in mean body mass index from 26.3 to 27.5 from 2012 to 2020. The publicly available birthweight data have limitations. Data collection evolved during the study period. The estimates for gestational age have become more accurate with first-trimester dating ultrasounds. In summary, birthweight is increasing among those born from 39 to 42 weeks of gestation. These increasing birthweights may be a factor in persistently high cesarean delivery rates despite national campaigns.⁵.