Exercise for a healthy-sized baby

Women who exercise regularly during pregnancy cut down their risk of having a heavy baby, regardless of their exercise status before pregnancy.

High blood sugar and pregnancy associated diabetes are risk factors for having a very large baby. Women who deliver excessively large babies, defined as newborns weighing in at more than 4 kg, no matter what their length, are at risk of a number of complications, such as heavy bleeding after delivery. These babies also have a higher risk of obesity later on in life.

To study the association between regular exercise before and during pregnancy and excessive newborn birth weight, researchers analysed data from the Norwegian Mother and Child Cohort Study and the Medical Birth Registry of Norway on 36,869 single pregnancies. Information on regular exercise was based on answers from two questionnaires distributed in pregnancy weeks 17 and 30. The main outcome measure was excessive newborn birth weight, defined as birth weight at or above the 90th percentile, which meant the top 10% of the newborns in terms of weight.

It was found that more than one in ten of the babies were too heavy, and more than half of these big infants were born to mothers who had already had children. Among women pregnant with a first child, those who reported exercising at least three times a week during their 17th week of pregnancy were about a quarter less likely to have an excessively heavy baby. Women who exercised at least three times weekly at 30 weeks' pregnancy were 22 percent less likely to have a too-big baby. Moreover, the amount of exercise women did before pregnancy was found to have no link with their chance of having an overly large newborn.

The above findings highlight the importance of exercise during pregnancy. Exercise could help prevent fetuses from growing too large by helping maintain the body's ability to keep blood sugar levels under control.