Pregnancy Weight Gain Linked to Serious Birth Complications

When Pregnancy Weight Gain exceeds the recommended range, the risk of life-threatening complications for the parent climbs. That is the warning from a massive new study of more than one million births, which argues that the rules expectant parents are given were written with babies in mind — not the people carrying them.

A research team at McMaster University in Ontario analyzed pregnancies recorded in the province's BORN Ontario birth registry, according to the university's Faculty of Health Sciences. The study, published October 22 in JAMA Network Open and covered by Medical Xpress, found that gaining more weight than recommended during pregnancy is linked to a higher risk of severe maternal complications — regardless of the person's starting weight.

Pregnancy weight gain recommendations have historically been built around the baby: hitting a healthy birth weight and avoiding preterm birth. This new research argues the rules deserve a second look through the lens of maternal health.

What the researchers found

Of the million-plus Ontario pregnancies studied, just over half of people gained more weight than guidelines recommend, while roughly one in five gained less than the recommended amount. Across that population, the researchers identified more than fifteen thousand cases of severe maternal morbidity — dangerous complications during the perinatal period.

Pregnancy Weight Gain above the recommended range raised the chance of severe complications across every pre-pregnancy body mass index category, with the sharpest increase seen in people who began pregnancy underweight.

The complications in question are the ones every obstetric team dreads: major bleeding, pre-eclampsia and admission to intensive care around childbirth. "We have long known that a person's weight before pregnancy is a risk factor," says lead author Sahar Khademioore, a Health Research Methodology PhD candidate at McMaster. "We wanted to know whether the amount of weight gained during pregnancy also influences the risk of complications."

Her answer, after sifting through one of the largest pregnancy datasets ever assembled for this question, is yes — and the effect holds no matter where a person started. "Because weight gain is something that can potentially be addressed, understanding how it relates to severe complications gives us an opportunity to better support maternal health," she adds.

The guidelines were written for babies

The current North American guidance on pregnancy weight gain was developed primarily to improve outcomes for newborns. Clinicians have focused on targets that reduce the odds of a baby being born too small or arriving too early, reported by the university.

The new findings suggest that framing is incomplete. When the recommendations were drafted, maternal health was treated as a secondary consideration — a gap this study now quantifies. The researchers argue that future updates to the guidance should weigh the parent's risk of severe complications alongside the baby's outcomes, rather than treating one as an afterthought.

That argument lands harder when set beside earlier work from the same institution. A separate McMaster-led study published this spring, part of the university's SERENE Project on severe maternal illness, found that nearly half of dangerous pregnancy complications occur outside the delivery room — during pregnancy itself or in the six weeks after birth, windows that standard surveillance barely monitors. Taken together, the two projects paint a picture of maternal risk that is broader in time and more tied to modifiable factors than guidelines assumed.

Why this matters beyond the numbers

For expectant parents, the takeaway is not to fear the scale — prenatal visits already track weight, and the study's lead author frames weight gain as something care teams can work with, not a verdict. But the research does change the conversation in the exam room: weight targets are no longer just about the baby's size, and counseling may need to explain the parent's own stakes.

There is also a systems implication. In many countries, maternal mortality reviews focus on what happens during labor and delivery. If severe complications track with gestational weight gain — a factor that is visible months before birth — then prenatal care programs that spot excessive gain early have a real chance to intervene, through nutrition support, closer monitoring, or earlier referral to maternal-fetal medicine specialists.

The finding also complicates a narrative that weight-related pregnancy advice only applies to people who start out overweight. Because the risk increase showed up in every weight category, including among those who began pregnancy underweight, clinicians may need to widen who gets intensive counseling rather than assuming thinner patients are low-risk.

What comes next

Guideline bodies move slowly, and this single study — large as it is — will not rewrite the recommendations overnight. But it gives policymakers and professional societies a concrete data point they did not have: a link between excess gestational weight gain and the most dangerous maternal outcomes, measured across a population bigger than most cities.

For readers, the health beat context is worth noting: maternal health research has historically been underfunded relative to its impact, and studies like this one — big, registry-based, focused on the parent — are exactly how that gap closes. You can read more of our health coverage on the link between sleep and depression subtypes, and on surprising findings about blood pressure, including a long-running food pantry experiment that lowered readings for years and new data showing most young adults with high blood pressure do not know they have it.