Vigorous activity may be linked to improved chance of pregnancy

Thursday May 10 2018

"Just 4 hours of vigorous exercise each week can boost a woman's chances of getting pregnant," reports the Mail Online.

A study of 1,214 women, who had previously had 1 or 2 miscarriages, found they were more likely to get pregnant during a 6-month period if they did more than 4 hours of vigorous physical activity a week.

Vigorous activity makes you breathe much harder and faster than normal. Examples include jogging or running, football and aerobics.

However, the study did not find an effect for any other activity level, such as low or moderate exercise. It's possible the finding was the result of chance rather than an effect of exercise, or that other unmeasured factors were involved.

The study does add to evidence that physical activity is generally healthy, including when you are hoping to get pregnant, and the researchers offered suggestions as to why vigorous exercise may aid fertility, such as by helping to reduce stress.

Unfortunately, the study did not look at whether physical activity influenced pregnancy outcomes in women who had previously had a miscarriage. For these women, being able to bring a baby to term may be more important than the time it takes to get pregnant again.

If you've been affected by miscarriage, you may find it helpful to read the Miscarriage Association's leaflet: Thinking about another pregnancy (PDF, 1.1Mb). It's important to remember that most miscarriages are a one-off and are followed by a healthy pregnancy.

Where did the story come from?

The study was carried out by researchers from the University of Massachusetts Amherst, the National Institute of Child Health and Human Development, and the University of Utah Health, all in the US.

It was funded by the Institute of Child Health and Human Development, and published in the peer-reviewed journal Human Reproduction on an open-access basis, so it's free to read online.

The report in the Mail Online was confused and confusing, suggesting that "a 10-minute walk is all that's needed" for overweight women to get pregnant, which is misleading. Overweight women who regularly walked for at least 10 minutes at a time each day (amounting to an average 3 hours a week) were more likely to get pregnant than overweight women who did not walk regularly – but this effect was not seen in women who were not overweight.

What kind of research was this?

This was an observational study of women who had previously had 1 or 2 miscarriages and were now trying to get pregnant again. Observational studies can be useful for spotting patterns, but they can't prove that a single factor (activity) directly causes another (pregnancy).

A further complication was that the women were actually recruited to a randomised controlled trial – looking at whether low-dose aspirin increased their chances of getting pregnant – where they had been randomised to receive aspirin or a placebo.

For the present study, the researchers re-analysed the data from that trial but grouped the women according to the amount of exercise they did. They said in their analysis that aspirin use did not affect the results, but they did not include this as a potential confounding factor in their main results. This reduces confidence in the findings.

What did the research involve?

At the beginning of the trial, researchers asked women to fill in a questionnaire measuring their physical activity over the past 7 days. The women were also weighed and measured, and gave other information about their health and lifestyle.

They were then followed up over 6 menstrual cycles, with regular pregnancy testing at least once a month.

The researchers looked to see whether women who reported differing levels of physical activity were more or less likely to have had a positive pregnancy test by the end of the 6 cycles, adjusting for factors such as age and marital status.

They also looked separately at women who were overweight or obese.

What were the basic results?

Among 1,214 women randomised in the trial, 797 (65.7%) became pregnant. These women were more likely to:

  • be a healthy weight according to their body mass index (BMI)
  • already have children
  • be married
  • be white
  • be more educated
  • have a higher income
  • not smoke
  • have had a shorter time between their last miscarriage and entering the trial

After taking these factors into account, researchers found women who did more than 4 hours of vigorous activity a week were 69% more likely to become pregnant than those who did no vigorous activity (odds ratio [OR] 1.69, 95% confidence interval [CI] 1.24 to 2.31).

However, the researchers found no increased chance of pregnancy related to:

  • less than 4 hours a week of vigorous activity
  • any amount of moderate activity
  • walking
  • sitting
  • low, medium or high total exercise levels over 7 days

The researchers then looked separately at these categories for women who were underweight or a normal weight, and overweight and obese (as defined by BMI).

They found that women who were considered to be underweight or a normal weight, who did more than 4 hours of vigorous activity per week, were 68% more likely to get pregnant than those who did none. However, there was no difference if they did up to 4 hours of vigorous activity a week.

In overweight or obese women, no amount of vigorous activity increased their chances of getting pregnant, compared to those who did none. However, the researchers did find that overweight or obese women who did moderate activity, for between 1 and 2 hours a week, were 58% more likely to get pregnant than those who did none (OR 1.58, 95% CI 1.03 to 2.42).

The researchers also looked more specifically at the impact of walking. They found that overweight or obese women who walked at least 10 minutes a day were 82% more likely to become pregnant than overweight or obese women who did not walk at least 10 minutes a day (OR 1.82, 95% CI 1.19 to 2.77). However, walking had no impact on the chance of getting pregnant for women who were not overweight.

How did the researchers interpret the results?

The researchers said their findings "provide positive evidence for the benefits of physical activity in women attempting pregnancy, especially those with higher BMI".

Conclusion

These results are difficult to interpret because they are conflicting.

It's hard to understand why vigorous physical activity may be beneficial when women do more than 4 hours a week but not if they only do 2 or 3 hours a week, for example, and why this effect was not found in overweight or obese women.

It's also hard to understand why walking for at least 10 minutes a day may be beneficial for overweight or obese women but not for those of a healthy weight (as defined by BMI).

The problem with carrying out lots of analyses on a single dataset, and then re-analysing with a different grouping, is that each additional calculation increases the likelihood of getting a positive result by chance alone.

When most of the results are negative – and the 2 positive results are the ones reported in the press – it makes you wonder if the study is really telling you anything useful.

There were other limitations too.

Women self-reported the amount of activity they did, and only at the start of the study, so we don't know how accurate the reports were or whether they continued that level of exercise during the next 6 months.

We also don't know anything about the women's diet, whether their weight changed during the study or about the fertility of their partner. All of these could have affected the chances of pregnancy.

Furthermore, observational studies cannot prove that the factor measured (activity) directly affects the results. It could be that related confounding factors, such as diet, played a part.

Overall, while the study may not tell us that much, it does add a little more weight to the evidence that it's beneficial to keep active throughout life, including while trying to get pregnant.

Read more about exercise in pregnancy.

Analysis by Bazian
Edited by NHS Choices