Association between obstetric interventions and gestational age


In a recent study, changes in gestational age distribution in the United States reflected increasing induction of labor rates between 1990 and 2017.

Association between obstetric interventions and gestational age | Image Credit: © kieferpix - © kieferpix -

Association between obstetric interventions and gestational age | Image Credit: © kieferpix - © kieferpix -

Changes in gestational age distributions for births in the United States are associated with changes in obstetric interventions, according to a recent study published in the Journal of Women’s Health.

A rise in obstetric monitoring and interventions has been observed for pregnancy and births in the United States. The rate of singleton first births between 37- and 39-weeks’ gestation with cesarean delivery increased 70% from 1990 to 2013, with an over 200% increase in induction of labor (IOL) for these births.

The reasons for increasing cesarean deliveries and IOLs is unclear, and evidence on potential risk factors is lacking. While elective IOL at 39 weeks has been associated with similar perinatal outcomes to expectant management, elective induction may also be associated with increased risk of adverse outcomes such as lower birth weight.

There is a possibility that increased rates of IOL and cesarean delivery are linked to shifts in gestational age distribution for US singleton first births. To determine the association between increased maternal interventions and changes in gestational age distribution, investigators conducted a population-based retrospective study.

Birth data from 1990 to 2017 was gathered from the National Vital Statistics Systems, and the sample was comprised of US singleton first births. Mothers were aged 15 to 49 years during birth and identified as non-Hispanic Black, non-Hispanic White, non-Hispanic Asian or Pacific Islander, and Hispanic.

Only cases with full information on gestational age, mother race, ethnicity, and age at birth, and obstetric intervention status were included. Data was combined for years 1990 to 1991, 1998 to 1999, 2006 to 2007, and 2016 to 2017.

Gestation was coded as under 34, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, and 44 or more weeks. Categories of obstetric intervention status included no IOL and vaginal delivery, no IOL and cesarean delivery, and IOL all deliveries.

Estimates were made for the probabilities of birth at each week of gestation in 1990, 1999, 2007, and 2017. Investigators plotted these probabilities separately based on race and ethnicity, then based on maternal age, and finally based on US state percentile. Joint probabilities for week of gestation and obstetric intervention status were then estimated.

A significant decrease in births occurring after 40-weeks’ gestation was seen from 1990 to 2017, from a rate of 0.282 to 0.172. In comparison, births from 37- to 39-weeks’ gestation increased from 0.385 in 1990 to 0.495 in 2017. Model gestation shifted from 40 weeks in 1990 to 39 weeks in 2007.

IOL rates increased from 0.117 of all births in 1990 to 0.313 of all births in 2017. Of this 0.196-proportion increase, 0.120 was attributed to births occurring earlier than 40 weeks. The risk of experiencing IOL increased more than 3 times among pregnant women, from 0.046 in 1990 to 0.165 in 2017.

While a significant difference in cesarean delivery with no IOL was not seen from 1990 to 2017, the rate briefly declined from 1990 to 1999 before increasing to a similar rate from 2000 to 2017. Rates of births after 40-weeks’ gestation and birth from 37- to 39-weeks’ gestation saw similar shifts across racial and ethnic groups and maternal age groups.

Overall, changes in gestational age distributions reflected shifts in IOL rates, indicating an association between obstetric interventions and gestational age distributions. Investigators recommended further research on the causes of increased obstetric interventions to examine the role of health care policy on gestational age distributions.


Masters RK, Tilstra AM, Coleman-Minahan K. Increases in obstetric interventions and changes in gestational age distributions of US births. Journal of Women’s Health. 2023. doi:10.1089/jwh.2022.0167

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