Behind the Headlines

Thursday December 4 2008

Mothers should not reduce their activity levels based on this report

“Exercise in pregnancy linked to fatal raised blood pressure condition,” The Daily Telegraph reported. The newspaper says exercise can raise the risk of developing pre-eclampsia, a condition where mothers have raised blood pressure and protein in the bloodstream shortly before or after birth.

The newspaper refers to a Danish study in which researchers used phone interviews to find out the exercise habits of over 85,000 pregnant women, and then examined medical records after birth. The researchers found a link between exercise and severe pre-eclampsia, but only for the most active mothers.

Despite what was reported in the newspaper, the researchers themselves state that current exercise recommendations should not be changed until more research produces similar findings. This is because the findings contradict previous research, only small numbers of women developed the condition and the pattern of the results was inconsistent.

Moderation and common sense should be applied to all forms of exercise, and mothers should not reduce their activity levels based on this report.

Where did the story come from?

This research was conducted by Doctor ML Osterdal from the Maternal Nutrition Group at the Statens Serum Institut in Copenhagen, along with other Danish and Norwegian colleagues. It was funded by several research foundations including the March of Dimes Birth Defects Foundation, a nonprofit organisation dedicated to improving the health of babies. It was published in the peer-reviewed medical journal, The British Journal of Obstetrics and Gynaecology.

What kind of scientific study was this?

This was a prospective cohort study looking at potential links between physical activity and pre-eclampsia.

Previous research had shown physical activity had a protective role during pregnancy. However, these previous studies were not prospective, meaning they did not collect data on pregnant women until pre-eclampsia had already occurred.

In this study, researchers decided to collect data on a large number of pregnant women early in pregnancy and further investigate any possible protective benefits from exercise.

The researchers enrolled over 100,000 women from general practices between 1996 and 2002. This was about 35% of the total pregnant Danish population at the time. Some of the women later miscarried or for other reasons were not available for analysis.

Detailed physical activity questions were asked at two telephone interviews at weeks 12 and 30 of pregnancy, although only the 12-week answers were included in this study. They asked about the usual duration and intensity of physical activity while pregnant, dividing the answers into seven groupings from zero minutes per week to 420 minutes or more a week (an hour or more a day).

The researchers categorised the intensity of exercise as vigorous, moderate or mixed according to the activity reported by the participants and an estimation of energy expended (the metabolic equivalent).

They also asked about other factors known or thought to influence the chance of developing pre-eclampsia. These include age, the number of previous pregnancies, height, body mass index (BMI), smoking, socio-economic position (grouped into six categories), marital/cohabitation status and type of residence. Complex statistical methods were used to adjust for these factors in their final analysis.

What were the results of the study?

Among the 85, 1389 women who completed the study, the two highest physical activity levels were associated with an increased risk of severe pre-eclampsia compared to the non-exercising group.

The adjusted figures suggest there was a 65% increase in risk for women who engaged in any intensity of physical activity for 270 to 419 minutes a week, and a 78% increase for those doing more than 420 minutes, compared to those who did no exercise.

The four other groups doing more moderate levels of physical activity (1 to 270 minutes per week) had no statistically significant association with risk of pre-eclampsia.

The adjusted chance for the two highest activity groups were reported in odds ratios of 1.65 (95% confidence interval: 1.11 to 2.43) and 1.78 (95% CI: 1.07 to 2.95).

What interpretations did the researchers draw from these results?

The researchers conclude that they were unable to find that leisure time physical activity had a protective effect against pre-eclampsia. They say that, “our data even suggests that leisure time physical activity exceeding 270 minutes/week in first trimester may increase risk of severe pre-eclampsia”.

What does the NHS Knowledge Service make of this study?

The findings of this research are ‘unexpected’, as the researchers have acknowledged. They say this unexpectedness makes it even more important to look at results of similar large studies from other countries. The researchers suggest that until this has happened, the recommendations remain unchanged.

There are several reasons to support the researchers’ cautious attitude to the findings:

  • The number of women in the two highest physical activity groupings was actually quite small: 2368 and 1240 in each. This compares to 53,984 women in the group who said they did no physical activity.
  • Out of the most active groups, only 44 women developed severe pre-eclampsia. While this is a statistically significant result, it may have arisen by chance in such a small group people.
  • Smoking is associated with changing the risk of pre-eclampsia, and so it was adjusted for during analysis. It is unclear whether this adjustment has fully neutralised the effect of this risk factor.
  • The researchers acknowledge that a misclassification of women into smokers and non-smokers may have occurred, especially if women changed their smoking habits during pregnancy.
  • There was no ‘dose response’ effect, meaning the risk of pre-eclampsia should go up as intensity and duration of activity increase. It is expected that if the link was genuine, a graded response to these factors would be seen in the results, but this was not apparent.

All these factors, the surprising nature of the finding and the lack of a biological explanation for the effect suggest that the researchers are correct in interpreting this result cautiously and calling for further studies. 

Analysis by Bazian

Edited by NHS Choices


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