The study found that the average age of pregnant individuals rose from 27.9 years in 2011 to 29.1 years in 2019, yet age accounted for only a small portion of the marked increase in adverse pregnancy outcomes seen during the same period.

Most striking, the rate of hypertensive disorders of pregnancy (high blood pressure with or without preeclampsia or eclampsia) rose by over 50% during the decade, yet the shift in age distribution of those giving birth accounted for less than 2% of that change.

“Although mothers are getting older at the time they deliver, that is not what’s causing these adverse birth outcomes,” said Zachary Hughes, MD, an internal medicine physician at Northwestern Memorial Hospital in Chicago and the study’s lead author. “What’s really driving it is pre-pregnancy health issues like diabetes and hypertension. That’s important to know because these are factors we could potentially modify.”

Using data from the National Center for Health Statistics Natality Files, researchers compared rates of preeclampsia and eclampsia, preterm birth and low birthweight among 3.9 million births in 2011 and 3.7 million births in 2019. The results showed moderate increases in preterm birth and low birthweight (which rose by about 2% and 4%, respectively) and larger increases in hypertensive disorders (a 52% increase). Using statistical methods to analyze the role of age in these changes, researchers found the shift in age distribution accounted for only a small portion of the increase across all outcomes assessed.

Adverse pregnancy outcomes have important health consequences, including an increased risk of heart disease both at the time of pregnancy and later in life—not only for the person giving birth but also for the baby. Researchers said preventing these adverse outcomes could help reduce cardiovascular risk throughout each lifespan.

“I hope these findings can lead more toward a culture change of focusing on health before pregnancy,” Hughes said. “A lot of people don’t establish care with a physician until they get pregnant, but really starting six months or a year before you’re starting to plan a pregnancy, that’s the time to maximize your health as much as possible and minimize the risk of adverse pregnancy outcomes. This also suggests that clinicians and policymakers could put more focus on pre-pregnancy health and managing things like hypertension and diabetes before someone gets pregnant.”

Across all age groups, people giving birth in their late teens to early thirties saw the steepest increases in adverse pregnancy outcomes. Further research is needed to determine which specific health issues—such as obesity, diabetes, hypertension or other factors—are driving these pregnancy complications in younger people, researchers said.

Current medical guidelines recognize pregnancy with anticipated delivery at age 35 or older as a risk factor for adverse outcomes and recommend additional screening and monitoring for these pregnancies. Based on these new study findings, researchers emphasized the importance of clinicians and people considering pregnancy, including those who are younger, to focus on addressing other health conditions to ensure the best possible chance of a healthy pregnancy and child. In addition, individuals diagnosed with gestational diabetes or high blood pressure during pregnancy should include these conditions as part of their health history going forward, even if the conditions resolve after birth, as they may lead to an increased risk of heart disease later in life.