Mothers who either gain or lose weight in between pregnancies could be putting both themselves and their unborn babies at risk, reported news sources including The Times , The Independent and BBC News.
The Daily Mail and The Daily Telegraph connected the study to the “celebrity trend” for crash-dieting after giving birth.
The stories were based on an editorial in the British Medical Journal and as such it is a presentation of the authors' expert opinions in the light of research into the issue of pre-pregnancy weight.
In the editorial, the authors cited previous studies on the effects of weight gain between pregnancies, which reportedly found effects on the baby and mother’s health to include higher blood pressure, risk of pre-eclampsia and increased infant birth weight. Weight loss was reported to be a risk for premature labour or low infant birth weight.
This appraisal is based solely on the editorial and this service has not reviewed the original studies. Therefore, we cannot draw conclusions on the original research from this paper. However, the opinion of the authors appears to be common sense, as does the message that maintaining a healthy weight is desirable if women are considering a future pregnancy.
Where did the story come from?
The editorial was written by Dr Jennifer Walsh and Dr Deirdre Murphy of the Women’s Hospital, Dublin, Republic of Ireland and published in the British Medical Journal . External peer reviewers have not looked at the article itself.
What kind of scientific study was this?
The authors have written their article based upon the findings of two cohort studies. The first is a large Swedish cohort of 207,534 women carried out between 1992 and 2001, that has looked at the link between changes in body mass index (BMI) between first birth and second pregnancy and how this affects maternal and infant outcomes. We are provided with no information about the second cohort study, other than that it was looking at whether there was a link between mothers’ nutrition and premature births.
What were the results of the study?
The authors say that the Swedish cohort found that when there was an increase in BMI of one to two units between the beginning of the first pregnancy and the beginning of the second, the risks of pregnancy induced diabetes, hypertension, pre-eclampsia, and large babies, increased twofold. With a three-unit increase in BMI the risk of stillbirth also increased.
The authors state that this study demonstrated that gaining weight increased the risk of complications in pregnancy and around the birth period, regardless of whether the mother was overweight or not.
The second study is reported to have found, conversely, that losing five or more BMI units between pregnancies put the mother at increased risk of having premature birth compared to women whose weight remained stable, or who gained weight. They said that this risk was greater if the woman had had a previous premature birth.
What interpretations did the researchers draw from these results?
The authors conclude that as pregnancy is a nutritionally demanding time for the expectant mother, women need to be aware of the implications that weight can have upon their own and their baby’s health.
They say that "although apparently conflicting", i.e. one advising against weight gain, the other against weight loss, the studies demonstrate the importance of maintaining a healthy weight before, during, and after pregnancy. In particular, women who have previously had poor outcomes around pregnancy or birth should try to obtain an optimal weight before planning another pregnancy.
What does the NHS Knowledge Service make of this study?
This editorial raises interesting questions around the issues of weight and pregnancy. Without assessing the reliability of the four references, we can make no firm conclusions on this subject as we do not know the size, methods, or reliability of these studies.
Also, although outcomes appear to have been measured in terms of BMI units gained or lost, we have no idea of the actual BMI of these women, whether they were underweight, normal weight or obese. Importantly, we also have no idea of the health of these women: there may have been several other factors that could have been affecting their outcomes, for example whether they were smokers, diabetics, or had additional medical or obstetric complications.
Reading the reports one might think that there is a conflict in the advice offered, suggesting that women planning a pregnancy shouldn’t gain weight, another that they shouldn’t lose weight. The opinion of the authors, which appears to be common sense, is that women should aim to achieve an optimal weight, with a healthy diet. The challenge is how to do this, and women should continue to receive individual health advice from their doctors as to the most appropriate course of action for them.