The obesity epidemic is unrelenting, especially among women. A study released this week found that more women than men are now affected by the disease, while the reverse was true just two years ago. The condition is associated with a wide variety of health risks, from chronic diseases to reproductive problems — and in a new study published June 7 in PLOS Medicine, researchers from the Karolinska Institutet in Sweden found that gaining weight between pregnancies further complicates mothers’ and infants’ health.

Already, research suggests that women who are overweight and obese during pregnancy face greater risk of certain complications, including gestational diabetes and preeclampsia. Infants born to obese women, on the other hand, sometimes score lower on the Apgar test, which is given no more than five minutes after babies are born to determine if they have breathing or heart trouble. While low scores can’t predict the future health of a child, they are commonly caused by asphyxia, a slippery slope that leads to additional adverse outcomes.

The researchers were interested to learn more about the ways a woman’s body mass index (BMI) could influence these complications and outcomes. They specifically looked at women’s BMI between their first and second pregnancies using data from the Swedish Medical Birth Register and any asphyxia-related outcomes: neonatal seizures and meconium aspiration syndrome, a severe breathing condition. Researchers also analyzed demographic information for more than 500,000 mothers who had their first and second live births between January, 1992, and December, 2012, including Apgar test scores and mothers’ and infants’ measurements and proportions.

Essentially, lead author Martina Persson and her team calculated weight gain and risks in two ways: one, they looked at low scores and asphyxia-related morbidities; and two, they looked at risks of these outcomes per every unit increase in a mother’s BMI.

The results showed an increased risk for all adverse outcomes in infants whose mothers’ BMIs increased by 2 units or more between pregnancies; 6 pounds is about one BMI unit for a woman of average height. These risks were more pronounced in women who started their first pregnancy at normal weight — a BMI of less than 25 — and then gained weight.

On the other hand, the increase in risk was less pronounced in women who were overweight and obese during early pregnancy. Persson and her colleagues speculate this may reflect the ways metabolism adapts to increased fat mass over time, meaning overweight women in early pregnancy might accumulate less fat in pregnancy than underweight and normal weight women.

To the researchers’ knowledge, this is the first time a study has assessed these particular risks and how they are influenced by change in weight over time.

It’s a strong study in that it analyzes a large number of births, exposures, and outcomes; however, the authors did not have information on gestational weight gain and do not know when exactly women gained weight between their first and second pregnancies. The distribution of fat may differ if weight is gained during or after pregnancy, they said.

Even so, they believe the findings are significant and could influence public health initiatives.

“Given the high prevalence of maternal overweight and the possible long-term consequences of birth asphyxia, our results have substantial public health relevance, as even modest weight increases in normal weight women may impact offspring outcomes on a population level,” the researchers concluded. Encouraging women to normalize BMI before pregnancy and to avoid weight gain between pregnancies, they said, would likely help improve infant health.

Source: Persson M, Johansson S, Cnattingius S. Interpregnancy Weight Change and Risks of Severe Birth-Asphyxia-Related Outcomes in Singleton Infants Born at Term: A Nationwide Swedish Cohort Study. PLOS Medicine. 2016.