It's not advanced age that is causing a rise in pregnancy complications, rather the poor health of the mother is to be blamed, suggests research.

The study led by a team from Northwestern Memorial Hospital in Chicago, US, showed that rising rates of adverse pregnancy outcomes, such as hypertensive disorders of pregnancy, preterm birth and low birth weight, over the past 10 years are largely attributable to the health status of a person before they get pregnant, rather than age.

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 per cent during the decade, yet the shift in the age distribution of those giving birth accounted for less than two per cent of that change.

"Although mothers are getting older at the time they deliver, that is not what's causing these adverse birth outcomes," said lead author Zachary Hughes, an internal medicine physician at Northwestern Memorial Hospital.

"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 two per cent and four per cent, respectively) and larger increases in hypertensive disorders (a 52 per cent increase).

Using statistical methods to analyse 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.

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.