Average Pregnancy Length in the US Is Shorter than in European Countries

Published on Thursday, 19 January 2023 Tweet

A new, multi-country analysis examining average pregnancy length and timing of birth in the US, England, and the Netherlands suggests that the US could improve maternity care outcomes by shifting from an obstetrics-centred model of care to one that favours less medical intervention during the birthing process.

Maternal health outcomes continue to worsen in the United States, where maternal and infant mortality rates far exceed rates in European countries and other wealthy nations. Associate Professor Rachel Rowe, from Oxford Population Health's National Perinatal Epidemiology Unit (NPEU), has collaborated on a new study led by Boston University School of Public Health (BUSPH) and Harvard Medical School, which is shedding insight on how hospital organisational structures and staffing within US maternity care may affect the birthing process and possibly contribute to adverse birth outcomes.

Published in the journal PLOS One, the study analysed gestational age patterns and timing of home and hospital births in three countries: the US, where most women are looked after by obstetricians, and England and the Netherlands, where midwives provide care for most women, with obstetric support when needed. Data from England came in part from the Birthplace study, which was led from the NPEU.

The findings showed that the average length of US pregnancies steadily declined by more than half a week between 1990 and 2020, from 39.1 weeks to 38.5 weeks, and that US pregnancies on average are shorter than pregnancies in England and the Netherlands. In 2020, only 23 percent of US births occurred at 40 or more weeks, compared to 44 percent of births in the Netherlands and 40 percent of births in England. The gestational age pattern for home births was the same in all three countries.

In all three countries, the researchers also examined birth timing by hour of the day for home births and vaginal births in hospital, and then repeated this analysis, limiting the comparison to hospital-based vaginal births without interventions, such as induction or labour augmentation, that could possibly alter the timing. In England and the Netherlands, births at home and in hospital occurred at similar times in the day, peaking in the early morning hours between 1 a.m.-6 a.m. But in the US, there was a noticeable difference in birth timing between the two settings: births at home peaked in the same early morning hours as home births in other countries, while the hospital-based births—even those with no interventions that could affect the natural pattern of timing—largely occurred during standard working hours for clinical staff, from 8 a.m. to 5 p.m.

The paper is the first international study to use large datasets to compare gestational age and birth timing in three high-income countries; most prior research has focused on data from individual hospitals or countries. The study included nationally representative and publicly available population-based birth data from all three countries, including data on more than 3.8 million births in the US and 156,000 births in the Netherlands in 2014, and more than 56,000 births in England from 2008-2010. The researchers examined home and hospital birth timing for births that occurred between 37 and 42 weeks.

Associate Professor Rachel Rowe, co-author from Oxford Population Health, said, “Dr Declercq and I first noticed similarities and difference in patterns of the timing of birth in the US and in our Birthplace data at a conference several years ago. It's gratifying to see these differences and others illuminated in this paper, as one possible contributing factor to the poorer birth outcomes we see in the US.”

“Our multi-country analysis shows that the US is an outlier in gestational age distribution and timing of low-intervention hospital births,” said study lead Dr. Eugene Declercq, Professor of Community Health Sciences at BUSPH. “There's a lesson to be learned from countries with more positive maternity outcomes than the US in having hospital staffing and operational plans conform more closely to the natural patterns of birth timing and gestational age rather than try to have birth timing fit organisational needs.”