Study Finds Birth Weight, Length of Pregnancy Declining in US


A review of more than 20 million births between 1990-2013 found average birth weight and average length of pregnancy has been declining.

Ryan Masters, PhD

Ryan Masters, PhD

Recent trends in delivery method have contributed to major changes in US birth weights and length of pregnancy, according to a new study.

The average birth weight and average length of pregnancy have been declining in recent decades, and data from a study conducted by investigators at the University of Colorado at Boulder link the change to increased rates of cesarean deliveries and inductions among US women.

Investigators, led by Ryan Masters, PhD, a social demographer with the University of Colorado at Boulder Institute of Behavioral Science, conducted an analysis using birth/infant death from the National Vital Statistics Systems. They sought to evaluate recent trends in obstetric practices, gestational age, and birth weight in the US.

Restricting the analysis to include only first-birth singletons among US-born non-Hispanic White, non-Hispanic Black, and Latina mothers, a cohort of 23,027,689 births from between 1990-2013 were identified for the analysis.

Masters and colleagues used life table techniques to analyze joint probabilities of gestational age-specific birth and gestational age-specific obstetric interventions, which includes induced cesarean delivery, induced vaginal delivery, not-induced cesarean delivery, and not-induced vaginal delivery, to detail trends in obstetric practices by gestational age.

Simulation techniques were used by investigators to estimate apparent counterfactual changes in birth weight distributions if obstetric practices had not changed between 1990-2013.

Between 1990-2013, the average birth weight of US first-birth singletons in the investigators' sample decreased 67 grams from 3314.5 grams to 3247.2 grams—among all births, the birthweight declined 69 grams from 3345.3 in 1990, to 3276.5 grams in 2013.

The average length of pregnancy decreased by an entire week from 1990-2013, with the average length decreasing from 40 weeks to 39 weeks. Additionally, fewer births stretched beyond 40 weeks and become more concentrated in the 37-39 week range.

Investigators noted multiple changes in obstetric practices during the observed time period. Among the 23 million-plus births included, the rate of cesarean deliveries rose from 25% in 1990, to 31.2% in 2013. Increases in rate of labor induction were also observed, with a rate of 12% in 1990, rising to 29% in 2013.

Investigators noted multiple limitations within their analyses, including being unable to measure changes in obstetric decision-making processes; data not allowing a measurement of reasons for the increased likelihood of the exposures; administrative data allowing to only measure changes in an exposure and outcomes from said exposure; and individual measures of maternal behaviors, characteristics, and other risk factors for obstetric interventions being limited.

"Our data indicate that there has been a dramatic shift in birth timing in this country, it is resulting in birthweight decline, and it is almost entirely due to changes in obstetric practices," Masters said in a statement.

This study, “Worth the Weight? Recent Trends in Obstetric Practices, Gestational Age, and Birth Weight in the United States,” was published online in Demography.

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