Physical activity should be encouraged during pregnancy, even for socially vulnerable women

Researchers at the University of São Paulo analyzed data for a group of pregnant women in western Amazonia. The analysis pointed to a correlation between at least 150 minutes of physical activity per week and lower birth weight, with less risk of childhood obesity and diabetes.

By Theo Ruprecht  |  Agência FAPESP – A Brazilian study reinforces the value and safety of leisure-time physical activity during pregnancy, even for socially vulnerable people. Using data for pregnant women in a city in Acre, a Brazilian state in western Amazonia, the researchers found that at least 150 minutes of exercise per week, as recommended by the World Health Organization (WHO), correlated with a reduction in the baby’s weight at birth, which is positive to avoid childhood obesity and diabetes, without increasing the risk of lower weight at birth than expected based on gestational age. 

An article reporting the findings is published in the journal PLOS ONE. The analysis was part of “Maternal and child health in Acre: birth cohort in the western Brazilian Amazon (MINA Brazil)”, a project in progress since 2015 with FAPESP’s support, and led by researchers at the University of São Paulo’s School of Public Health (FSP-USP).

“Previous studies investigated how leisure-time physical activity during pregnancy affects birth weight and other outcomes, but there was a lack of prospective research on the topic in low- and middle-income countries,” said Marly Augusto Cardoso, a professor in the Nutrition Department at FSP-USP and principal investigator for the project. 

According to Cardoso, ethnic differences may influence the results. In contrast with other studies, in this case about 80% of participants placed themselves in categories other than White. In addition, specific traits of deprived areas may enter the equation. In rich countries, for example, concern about the weight of newborns is limited to overweight, which is only one extreme, whereas in low-income regions maternal and child malnutrition is the main worry, and this can be a factor in low birth weight. Also, women in poor regions tend to be more physically active at work or at home. “In this context, does leisure-time physical activity increase the proportion of small-for-gestational-age babies?” Cardoso wondered.

The MINA Brazil study sample displayed both extremes – underweight and overweight at birth – and this answered the question, she added. As noted in the article, this is the first prospective study conducted in a middle-income country with non-white women in the majority to examine the effect of leisure-time physical activity during pregnancy on birth weight.

Data collection

The population studied by the FSP-USP group lives in Cruzeiro do Sul, which has some 88,000 inhabitants and is in Acre state. Pregnant women in the town were monitored between February 2015 and January 2016, and the babies born to them were also assessed with the consent of their families. The participants were examined and answered questionnaires on a range of topics from lifestyle, child nutrition and gut microbiota to infection by malaria. “We want to address different questions with this cohort,” Cardoso said.

For this article specifically, the researchers focused on data regarding leisure-time physical activity for 500 volunteers covered by the project. The women reported the amount of activity undertaken in the second and third trimesters of pregnancy and were then weighed and categorized according to whether they had reached the recommended minimum of 150 minutes of exercise per week. Babies were weighed at birth. 

The recommended minimum of leisure-time activity was performed by only 7.3% of the women in the first trimester and 9.5% in the second. Before pregnancy, the proportion was 42%. “Unfortunately, this isn’t peculiar to Cruzeiro do Sul. Even now many physicians recommend rest during pregnancy, especially in the first trimester,” said Maíra Malta, first author of the article. Malta is a professor in the program of graduate studies in collective health at the Catholic University of Santos (UNISANTOS) in São Paulo state. “However, we have strong and solid evidence of the benefits of leisure-time physical activity during pregnancy for both mother and baby.”

Adequate weight

The habit of performing at least 150 minutes of physical activity per week in the third trimester of pregnancy was found to correlate with an average reduction of 137.9 g in birth weight. However, the proportion of low-weight babies born to these women did not rise as a result. “This may mean physical activity lowers the risk of excess birth weight without leading to the opposite extreme,” Malta surmised. 

The article also concludes that this effect was partly mediated by the mothers’ excessive weight gain during pregnancy. In other words, the result obtained was probably due to some extent to the fact that exercise helps prevent pregnant women become overweight or obese. Because women who put on more weight during pregnancy tend to give birth to larger babies, adequate management of maternal weight by means of physical activity may explain part – but not all – of the babies’ reduced birth weight. 

“It was a small mediating effect,” Malta explained. Previous research showed that the correlation remained significant after adjustment for maternal body mass index (BMI), suggesting that the effect of physical activity on birth weight is only partly mediated by maternal weight.

The findings were different for women who did perform the recommended amount of leisure-time physical activity during the second trimester of pregnancy. “Our study underscores the need for health workers to advise pregnant women to get exercise, especially if they show a tendency to put on weight during the period,” Cardoso said. “Of course, an antenatal obstetric assessment is essential, as rest is indeed required in some cases, but we must reduce the proportion of pregnant women who are sedentary.” 

Limitations and future plans

MINA Brazil is part of an international collaboration called The Gestational Weight Gain (GWG) Pooling Project Consortium, which is supported by the Bill & Melinda Gates Foundation. It investigates the impact of GWG on maternal and child health in low- and middle-income countries.

Few pregnant women in the cohort performed the recommended 150 minutes of physical activity per week, so it was not possible to evaluate the effect of exercise on birth weight or other outcomes. For the same reason, the researchers were also unable to find out exactly how the time devoted to physical activity correlates with its benefits. 

In addition, the study sample did not include pregnant women living in rural areas, who could have presented other peculiarities. Future research should be conducted in these directions, the researchers say in the article. They will continue to follow these women and their children to measure the medium- to long-term effects of physical activity. In parallel, the project will investigate different aspects of health and lifestyle, such as recurring exposure to malaria, which previous studies have shown to increase the risk of anemia in the baby’s early years (more at: agencia.fapesp.br/37047/). 

The article “Leisure-time physical activity in Amazonian pregnant women and offspring birth weight: A prospective cohort study” is at: journals.plos.org/plosone/article?id=10.1371/journal.pone.0265164.