Thursday September 27 2007
Too early to say that strenuous exercise causes miscarriages
Pregnant women who do strenuous exercise, such as jogging or playing racket sports and ball games, more than treble their risk of miscarriage, newspapers reported.
The newspapers said vigorous high-impact exercise within the first few months of pregnancy was associated with the highest risk of miscarriage. Exercise in the later stages of pregnancy did not affect the risk.
The reports are based on a study on more than 92,000 women in Denmark that looked for a relationship between exercise and miscarriage. The study performed different analyses on the results, which resulted in conflicting findings. As a result, the researchers are themselves cautious about suggesting a link between exercise and miscarriage.
The relationship between exercise and miscarriage has not been proven beyond reasonable doubt by this study.
Where did the story come from?
Dr Madsen and colleagues mainly from the Department of Child Health, at the National Institute of Public Health in Copenhagen, Denmark conducted this study. The initial collection of data in the cohort study was supported by the Danish National Research Foundation, this study was also supported by the Danish Medical Research Council.
The study was published in the peer-reviewed medical journal the British Journal of Obstetrics and Gynaecology.
What kind of scientific study was this?
The study was an analysis of both prospective and retrospective data collected in a large cohort study.
The study analysed the data from 92,671 pregnant women to look at the association between “leisure time physical activity”, amongst other details, with miscarriage in women who became pregnant between 1996 and 2002.
The women were enrolled during their first antenatal visit to their GP and then given a “computer-assisted telephone interview” at 12 to 16 weeks of their pregnancy. The interview asked the women if they engaged in any exercise, what type of exercise this was, and how much of these types of exercise they took.
The researchers then looked at various registry databases or contacted the women in person to find out what the outcome of the women’s pregnancies were.
The study found that 3,187 women miscarried before 22 weeks gestation. Because of the timing of the exercise interview (after the 12th to 16th week period in which most miscarriages occur) the study collected exercise data from 741 women while the outcome of the pregnancy was unknown (prospectively), and from 2,446 others after they had experienced a miscarriage (retrospectively).
Women who had already miscarried by the time of the interview were asked similar questions about exercise to those who were still pregnant.
What were the results of the study?
Almost half of the women had exercised during pregnancy and common types of exercise were low-impact exercise such as bicycling, horse riding or swimming.
The relative chance of miscarriage was calculated for all the women according to how long they usually spent exercising during the week. The authors found that they obtained different results if they looked at the prospective and retrospective data together, compared to the prospective data alone.
Analysis of the prospective data alone (data from women who had answered their telephone questionnaire before outcome of the study was known) did not show any significant associations between exercise and miscarriage. However if all the data was analysed together they found that increasing amounts of exercise correlated to an increased risk of miscarriage.
What interpretations did the researchers draw from these results?
The authors were very cautious about drawing any firm conclusions from this study as the difference between the results when analysed retrospectively and prospectively suggest that their results might have been affected by recall bias.
This bias could have occurred if the women who had already miscarried remembered or relayed details of their activity in a different way to women who had not miscarried.
The authors say “the association between exercise and risk of miscarriage need not necessarily reflect a causal mechanism” also pointing out that nausea is less common in pregnancy that ends in miscarriage and that women may stop exercising in early pregnancy if they fell nauseous.
They also conclude by saying that “in spite of the findings of this study, we do, however, think that it is too early to draw any public health inferences on this basis. Many positive effects of exercise are well established, and the findings of this study need to be replicated.”
What does the NHS Knowledge Service make of this study?
This is study is based on a large collection of data which has been analysed in several different ways.
The relationship between exercise and miscarriage has not been proven beyond reasonable doubt by this study, because of several features of the data which the authors acknowledge:
- A significant association between exercise and miscarriage was only found when the researchers pooled retrospectively and prospectively collected data. A different result was obtained when an analysis was performed on only the prospective data, which suggests that women may be attributing their miscarriage to their exercise when asked possibly leading questions by telephone.
- There is also some doubt over the finding that women who exercised more and who engaged in the most vigorous activities were most at risk, and those who did more gentle exercise such as swimming did not raise their chances of a miscarriage. This is also subject to this recall bias and no such effect is shown in the data collected prospectively.
- Factors which are commonly associated with pregnancy, such as nausea, could also have a confounding affect if they influence a women’s usual exercise routine. The researchers point out nausea is less common in pregnancy that ends in miscarriage and that women may stop exercising in early pregnancy if they feel nauseous.
The authors and newspapers sensibly call for confirmatory studies before drawing any public health inferences from the data.
Sir Muir Gray adds…
The aim of exercise is to improve the four Ss; Strength, Stamina, Suppleness and Skill. At any age and in any condition these can be achieved without high impact, which always carries a risk. Pregnant women should exercise to maintain and improve the four Ss, but that need not involve high impact activities.