Many women are annoyed by their "baby weight," or the weight they gained during pregnancy. According to new research, though, gaining even a moderate amount of weight between first and second pregnancy can pose fatal health risks to newborns.

A study involving over 450,000 women suggests gaining weight between pregnancies increases the risk of the baby dying in its first year of life, and increases the risk of stillbirth. On the opposite end of the spectrum, weight loss between pregnancies reduced the likelihood of neonatal death in babies of overweight women. Study author Sven Cnattingius, reproductive epidemiology professor at the Karolina Instituet in Stockholm, said in a press release the implications for public health are "profound."

He continued: "Around a fifth of women in our study gained enough weight between pregnancies to increase their risk of stillbirth by 30-50 percent, if they had a health weight during their first pregnancy."

The study was based off data from the Swedish Medical Birth Register, which collected data from women who gave birth to their first and second children between 1992 and 2012. The findings showed that babies of mothers who gained over 4 BMI (body mass index) units — about 11 kilograms for a Swedish woman of average height — between pregnancies had about 50 percent greater risk of their baby dying within 4 weeks of life than women who were of stable weight.

For mothers with a normal BMI during their first pregnancy, infant death was 27 percent greater for women who gained between 2 and 4 BMI points and 60 percent higher for mothers who gained 4 BMI units or more. In contrast, women who lose at least 6 kg before their second pregnancy had about a 50 percent reduced risk of neonatal death.

"The prevalence of overweight and obesity in pregnant women has reached epidemic levels," said study co-author Eduardo Villamor, professor of epidemiology at the University of Michigan School of Public Health. "More than half of women in the [U.S.] and one in three women in Sweden are either overweight or obese at the start of their pregnancy. Our study findings highlight the importance of educating women about maintaining a healthy weight during pregnancy and reducing excess weight before becoming pregnant as a way to improve infant survival."

Source: Cnattingius S, Villamor E. Weight change between successive pregnancies and risks of stillbirth and infant mortality: a nationwide cohort study. The Lancet. 2015.