Pregnancy and child

Pregnancy: weight control cuts complications

“Dieting in pregnancy is good for you,” according to The Independent, while the Daily Mail has warned pregnant women not to eat for two since “piling on the pounds during pregnancy” increases the risk of complications.

Both these news stories are based on a study that compared ways to manage weight during pregnancy, but did not tell women to diet or look at the effects of overeating, as the headlines implied. Instead, the research reviewed previous studies to look at how diet, exercise or a combination of the two affected maternal weight gain and the risk of health problems for babies. In particular, it found that compared to other interventions such as exercise, following a diet plan (but not a weight-loss diet) during pregnancy was more effective at reducing the amount of weight mothers gained. This had no adverse effect on the baby and reduced the risk of pre-eclampsia, diabetes, high blood pressure and premature birth.

This large study comes in the wake of concerns about the growing problem of obesity in pregnancy, which can cause serious problems for the mother and is a risk factor for later obesity in the child. It has found that dieting during pregnancy to maintain a healthy weight is safe, effective and has no effect on the baby’s birth weight, a factor which many woman worry about.

Currently, pregnant women are advised not to “eat for two” or reduce their calories, but to follow a healthy, varied diet with plenty of fruit and vegetables and a minimal intake of foods that are high in fat and sugar. Women who suspect they are overweight or obese are advised to talk to a dietitian, who will help them with a weight management programme.

Where did the story come from?

The study was carried out by researchers from several institutions in Europe, including Queen Mary University of London and the University of Birmingham. It was funded by the National Institute for Health Research’s Health Technology Assessment Programme. It was published in the peer-reviewed British Medical Journal.

Predictably, many newspapers made a meal of reporting this research, warning women not to “eat for two” even though women have been advised against doing this for several years now. The Metro’s headline that expectant mothers were being “urged to go on a diet” was also misleading. The study did not advise all women to follow a calorie-controlled diet but instead suggested that dietary interventions should be targeted at women who are obese or overweight. The paper’s photo of a pregnant woman holding weights was also misleading, since the study found diet to be more effective than exercise at reducing weight in pregnancy.

What kind of research was this?

This meta-analysis combined the results of randomised controlled trials which had looked at the effects of diet, exercise or a combination of the two on weight gain in pregnancy. Researchers also explored whether such interventions had any other effects during pregnancy and birth, and whether they affected the weight of the baby.

The researchers point out that obesity is a “growing threat” to women of childbearing age, with half the population being either overweight or obese. In Europe and the US, 20–40% of women gain more than the recommended weight during pregnancy. The researchers say that excessive weight gain during pregnancy is associated with adverse pregnancy outcomes, while for the children maternal obesity is a risk factor for obesity during childhood, which can persist into adulthood.

The authors argue that there is a need to identify safe and effective ways to help women manage their weight during pregnancy.

What did the research involve?

The authors analysed the results of 44 randomised controlled trials involving over 7,000 women.

They conducted searches of several electronic databases to find trials on the subject of pregnancy and weight. They also searched for relevant unpublished studies in sources of information such as conference databases. From these, they selected randomised controlled trials that tested the effects of dietary or lifestyle interventions on maternal and baby weight, as well as maternal and foetal outcomes.

The interventions in the trials were classified into three groups: mainly diet-based, physical activity-based, or based on both diet and physical activity. Studies were assessed for the quality of their design and methods to minimise the risk of bias.

The main outcome assessed was weight-related changes in the mother and baby, but researchers also looked at whether diet or exercise were associated with the risk of other critical pregnancy outcomes, including gestational diabetes, pre-eclampsia (a dangerous complication of pregnancy), premature delivery, stillbirth and shoulder dystocia (an emergency during childbirth where one of the baby’s shoulders becomes stuck behind the mother’s pubic bone). They summarised the strength of the evidence for these outcomes using an established system for grading evidence.

To explore possible further adverse effects, they undertook a separate search and review of the safety of diet and exercise in pregnancy, based on established methods. They analysed the data from the selected trials using standard statistical methods.

What were the basic results?

The researchers’ analysis included 44 randomised controlled trials involving 7,278 women, looking at the effects of diet, exercise or a combination of the two.

The researchers compared the outcomes seen in women who were assigned interventions and women in control groups (who were not offered any interventions). They found that:

  • Women who dieted, exercised or did both gained on average 1.42kg less than women in the control groups (95% confidence interval [CI] 0.95 to 1.89kg).
  • Dieting, exercising, or doing both had no significant effect on the baby’s birth weight (mean difference -50g, 95% CI -100 to 0g), or whether babies were large or small for gestational age (the amount of time they had been in the womb).
  • On its own, physical activity was associated with a reduced birth weight of 60g on average (95% CI -120 to -10g).
  • Diet, exercise, or both reduced the risk of pre-eclampsia (relative risk [RR] 0.74, 95% CI 0.60 to 0.92) and shoulder dystocia (RR 0.39, 95% CI 0.22 to 0.70), with no significant effect on other critically important outcomes.
  • Dietary intervention resulted in the largest reduction in mothers’ weight gain during pregnancy. Compared with controls, women following dietary interventions were 3.84kg lighter and had better pregnancy outcomes than with other interventions (95% CI 2.45 to 5.22kg).

The overall evidence rating for the underlying studies was reported as low to very low for important outcomes such as pre-eclampsia, gestational diabetes, gestational hypertension and preterm delivery.

How did the researchers interpret the results?

The researchers concluded that diet and exercise can reduce maternal weight gain and improve outcomes for both mother and baby, with dietary intervention being the most effective. The diets in the trials included:

  • a conventional balanced diet (based on an energy intake of 18–24kJ per kg of body weight)
  • a low-glycaemic diet with unprocessed whole grains, fruits, beans and vegetables
  • a diet with a maximum of 30% fat, 15–20% protein and 50–55% carbohydrate

Based on their findings, the researchers suggest that regular advice on planned nutritional intake should be provided to women from early pregnancy onwards, targeting overweight and obese women, who they say would benefit most.


This study has found that dieting during pregnancy to maintain a healthy weight is safe, effective and has no consequential effect on the baby’s birth weight, a factor which many women worry about.

It’s important to correct some of the inaccurate news coverage of this research. The research highlights the importance of eating healthily during pregnancy, but does not mean that all pregnant women should be put on diets. Nor does it recommend a reversal of the current advice that women should not eat for two, which has long been discouraged.

While putting on too much weight can affect a woman’s health and increase the risk of complications, gaining too little weight can also cause problems and mean the body is not storing enough fat. The current advice is not to go on a weight-loss or calorie-restricted diet during pregnancy, although a woman’s midwife or GP may have special advice if she weighs more than 100kg. Instead, current advice is based on eating a balanced diet and managing weight at an appropriate level. While it’s unlikely to make juicy headlines, the simple fact is that women should eat a normal amount and a balanced range of nutrients.

Weight gain in pregnancy varies greatly, although most pregnant women can expect to gain 8–14kg, most of it after week 20, as the baby grows and the body lays down enough fat to make breast milk after the baby is born. The medical team supporting a woman during pregnancy will monitor her changes in weight and diet, and will make appropriate suggestions to help her and her baby be as healthy as possible.

NHS Attribution