"Maternal weight gain raises stillbirth risk", reports The Guardian, and other media outlets.
A large study in Sweden found women whose weight increased by more than two BMI units (about 6kg for a woman who is 1.67m tall) between their first and second pregnancy had a higher chance of having a stillbirth. Some of these women had a higher chance of a baby dying within the first year after birth.
The study showed that for women who gained a significant amount of weight – four BMI units, or approximately 11kg for a woman who is 1.67m tall – the risk of a stillbirth was more than 50% higher than for women whose weight remained stable between pregnancies.
Although this study found an association between weight gain from one pregnancy to the next, and an increased risk of stillbirth and death in the first year after birth, it cannot prove weight gain was the cause of the stillbirths or infant deaths.
The risk of stillbirth and infant death should also be put in context. Overall levels of stillbirth and infant deaths in the study were low. Even with the increased risk linked to weight gain, the absolute risk of a stillbirth or infant death remains low.
However, the study adds to previous research that shows women who are overweight tend to have more problems in pregnancy. Keeping to a healthy weight, and losing weight if you need to, is a good way to prepare for a healthy pregnancy and birth.
Where did the story come from?
The study was carried out by researchers from the Karolinska Institutet in Sweden and the University of Michigan School of Public Health in the US, and was funded by the Swedish Research Council for Health, Working Life and Welfare and the Karolinska Institutet.
It was published in the peer-reviewed medical journal, The Lancet, and you can read an abstract or summary online.
The study was covered by The Guardian, The Daily Telegraph and the Mail Online, mostly accurately. However, the reports did not point out this study cannot prove the extra stillbirths and infant deaths were caused by women's weight gain.
The Daily Mirror went for a more inflammatory tone, saying: "Mums who gain as little as a stone after the birth of their first child put their second baby at greater risk of dying".
The study does not show this is the case, and this message could cause undue distress to women who are either pregnant or who have lost a baby in the past.
What kind of research was this?
The researchers conducted a population-based cohort study of more than 450,000 women in Sweden.
Cohort studies are a good way of comparing how certain factors are linked to certain outcomes – in this case, how weight gain between pregnancies is related to stillbirth or infant death.
However, they cannot rule out the chance the findings are the result of other factors the researchers have not taken into account.
What did the research involve?
The study was based on records of pregnancies, births and deaths in Sweden over a 20-year period. Researchers studied the records of all women in the Swedish Medical Birth Register who had their first two births between January 1 1992 and December 31 2012.
They looked at whether there was a difference between the women's weight as measured in the first three months of their first pregnancy, and their weight measured at the same point in the second pregnancy.
They then checked to see which pregnancies had resulted in a stillbirth or an infant death in the first year after birth, and whether weight gain was linked to stillbirth or deaths, after allowing for other factors.
The researchers had complete information about more than three-quarters of the women who had their first and second child during this period.
They adjusted their figures to take account of factors, such as the mother's age at second pregnancy, her level of education, the amount of time between the pregnancies, and whether she smoked during either pregnancy.
The researchers calculated the risks of having a stillbirth or infant death for women who had gained weight between pregnancies, compared with the risks for women whose weight remained more or less unchanged between pregnancies (a change of one BMI unit or less between pregnancies).
The research also looked into the effects of different amounts of weight gain, as well as the link between weight loss and stillbirth or infant deaths.
What were the basic results?
For all women, a weight gain of two BMI units (approximately 6kg for a woman who is 1.67m tall) or more between pregnancies was linked to an increase in the risk of stillbirth in the second pregnancy.
For women who gained two to four BMI units, the risk increased by 38% (relative risk [RR] 1.38, 95% confidence interval [CI]), while it rose by 55% (RR 1.55, 95% CI) for women who gained four BMI units or more.
When the researchers looked at the link between weight gain from one pregnancy to another and infant mortality (when a baby dies within the first year of life), they found a different pattern for women who were a healthy weight in their first pregnancy and those who were overweight in their first pregnancy.
Only women who were a healthy weight in their first pregnancy and then put on weight before their second pregnancy had an increased risk of infant mortality in the second pregnancy. This increased risk ranged from 27% (RR 1.27, 95% CI 1.01-1.59) to 60% (1.60, 1.16-2.22) based on how much weight they'd put on.
Women who were already overweight in their first pregnancy and then gained more weight before their second pregnancy did not have a higher risk.
Looking at the effect of weight loss, the study found those women who were overweight in their first pregnancy and lost weight by their second pregnancy had a lower chance of having a neonatal death (within the first 28 days of life) second time around.
Women who were normal weight and lost two BMI units of weight had a higher chance of an infant death, possibly because weight loss in women who are not overweight could be the result of illness.
How did the researchers interpret the results?
The researchers said their findings had "substantial public health implications".
They said there were a number of reasons why gaining weight might increase the chances of stillbirth or infant death, including an increase in inflammation and blood pressure triggered by gaining body fat.
Women who are overweight are also more likely to have problems during the birth itself, which can increase the chances of infant death.
They said reducing the numbers of overweight mothers and discouraging weight gain between pregnancies "might reduce stillbirth and infant mortality risks", but "the pathways by which overweight and obesity affect stillbirth and infant mortality are still to be established".
This study has a number of strengths. Firstly, it covers a large number of births thanks to the Swedish system of recording pregnancies, births and deaths. The researchers took account of several factors that could have skewed their results, such as the mothers' age at second birth and whether or not they smoked.
The results show risk rose in line with the amount of weight gained, which strengthens the chances this finding represents a causal relationship.
But cohort studies can never prove cause and effect. Weight gain might be a marker for something else that affected the outcomes.
For example, the women who gained most weight might be generally unhealthier, have a poorer diet, or be more likely to drink alcohol in pregnancy than those who stayed at a steady weight. These factors were not recorded, so we don't know.
However, the study adds to previous research, which shows that women who are overweight tend to have more problems in pregnancy, such as gestational diabetes, pre-eclampsia and difficulties in childbirth.
Keeping to a healthy weight, and losing weight if you need to, is a good way to prepare for a healthy pregnancy and birth. Find out more about what you can do to have a healthy pregnancy.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 2 December 2015
Daily Mirror, 2 December 2015
The Telegraph, 3 December 2015
Mail Online, 3 December 2015
Links to the science
The Lancet. Published online December 2 2015