Pregnancy and child

Exercise in pregnancy and baby size

“Aerobic exercise during pregnancy ‘produces lighter babies’,” reported The Times . It said researchers have found that women who trained on exercise bikes for 40 minutes up to five times a week had babies who were 143g lighter on average than babies of women who did not exercise.

This relatively small study used a good study design to investigate this question, randomly assigning 98 pregnant women to a personalised cycling programme or a group that did not cycle. Cycling did not affect the women’s BMI or glucose metabolism, but it did affect their babies’ birth weight.

Although newspapers have suggested that these lighter babies may be less “obesity prone”, it is not possible to say from this study what the longer-term effects are on the children's weight. This is because the babies were only assessed at birth.

Ideally, these results should be confirmed in larger studies. A few other small studies looking at non-weight-bearing exercise in pregnancy have not found the same effect on birth size.

Where did the story come from?

Sarah A Hopkins and colleagues from the University of Auckland and Northern Arizona University carried out this research. The first author of the study was funded by the National Research Centre for Growth and Development and by an unrestricted grant from Novo Nordisk, a pharmaceutical company. The study was published in the peer-reviewed Journal of Clinical Endocrinology and Metabolism.

The Times, Daily Mail, BBC News and Daily Mirror covered this story. They accurately reported the differences in weight seen in the babies in the two groups.

What kind of research was this?

This randomised controlled trial (RCT) investigated the effect of exercise in pregnancy on maternal metabolic factors and outcomes in the newborn babies. The researchers reported that the few RCTs that have been carried out to date have mainly looked at high-impact, weight-bearing exercise and produced inconsistent findings regarding its effects on offspring’s birth size. This study examined the effects of cycling, which is a non-weight-bearing exercise.

A randomised controlled trial is the best way to determine the effects of a particular intervention. Randomly assigning individuals into groups should balance the groups for factors that could affect the results, in which case any resulting differences can be attributed to the interventions that each group received.

What did the research involve?

The researchers enrolled 98 women aged between 20 and 40 years old who were pregnant with their first baby. They then randomly assigned the women to either a cycling programme or no cycling (control group). Women who smoked, drank alcohol or were having more than one baby could not take part. The members of the cycling group were each given a personalised exercise bike training programme between 20 weeks of pregnancy and delivery. The researchers then looked at whether the cycling group differed from the non-cycling group in terms of maternal insulin resistance and newborn characteristics, including size.

The cycling programme involved up to five 40-minute sessions a week. The women were asked to continue doing this until at least 36 weeks into their pregnancy, after which they were encouraged to do as much of their programme as they could manage. Women recorded their cycling and heart rate in an exercise diary. Every two weeks, the women took part in a monitored exercise session, during which their heart rate and blood pressure were measured. Their aerobic fitness was also measured at the start of the study and in late pregnancy (at about 35 weeks).

Insulin resistance is a condition in which cells do not respond appropriately to insulin. Therefore, levels of glucose in the blood can become higher than normal. The risk of insulin resistance is reported to increase in late pregnancy, in some cases leading to gestational diabetes. The women in this study had their insulin sensitivity measured 19 weeks into their pregnancy and at 34-36 weeks. The birth weight, length and head circumference of the participants’ babies were also recorded.

What were the basic results?

Of the 98 women recruited, 84 (86%) had full data on their own and their babies’ outcomes available and were included in the analysis. The analysed groups (cycling and control) were similar, though the cycling group was slightly older with an average age of 31 years compared with 29 years in the control group. On average, women in the cycling group completed 75% of their prescribed exercise.

Cycling did not significantly affect maternal BMI, body weight or insulin sensitivity in late pregnancy, but was associated with increased aerobic fitness compared to the control group. Cycling did not affect the length of pregnancy. Babies of women in the cycling group were 143g lighter on average than babies of women in the control group and had lower BMIs. This difference remained after length of pregnancy and gender of the baby were taken into account. Babies in the two groups did not differ in their length and had similar percentages of body fat when they were given a postnatal scan.

How did the researchers interpret the results?

The researchers concluded that regular moderate intensity, non-weight-bearing exercise in the second half of pregnancy was associated with lower offspring birth weight, but did not affect maternal glucose metabolism. They say that further studies are required to validate their findings in other populations.


Points to note when interpreting this study include:

  • The results may have been affected by the fact that not all of the women who were randomised were analysed and that more women withdrew from the control group (12 women) than the cycling group (2 women). However, the researchers say that this should not matter and that there were no differences between women who did and did not complete the study.
  • The study was relatively small and ideally should be confirmed by larger studies. This is particularly the case as the researchers report that the few other studies that have looked at non-weight-bearing exercise in pregnancy have been small and have not shown an effect on birth size.
  • Only late-pregnancy maternal and newborn outcomes were assessed. As such, the study cannot tell what the longer-term outcomes for the mother or child might be.

Current NICE recommendations on antenatal care advise that:

  • Pregnant women should be informed that beginning or continuing a moderate course of exercise during pregnancy is not associated with known adverse outcomes.
  • Pregnant women should be informed of the potential dangers of certain activities during pregnancy, such as contact sports, high-impact sports and vigorous racquet sports that may involve the risk of abdominal trauma, falls or excessive joint stress, and scuba diving, which may result in foetal birth defects and foetal decompression disease.

NHS Attribution