Physical exercise during pregnancy and reproductive outcomes
PhD thesis on Physical exercise during pregnancy
Physical activity is of crucial importance in the prevention of a number of diseases and is therefore given high priority in the treatment of illnesses and in health promotion, including antenatal care, where physical activity during pregnancy is considered to have a positive health effect on several pregnancy outcomes, e.g. pre-eclampsia and gestational diabetes. Moreover, pregnancy is often followed by a persistent weight increase, which is assumed reducible by means of physical activity during pregnancy. On the basis of these assumptions, current guidelines in Denmark and a number of other countries today recommend pregnant women to be physically active at the same level as the non-pregnant population. The Danish guidelines comprise at least 30 minutes of moderate physical activity per day. However, only limited scientific evidence exists on the influence of the mother’s level of activity on the health and development of the foetus. The sparse evidence regarding foetal health, together with recent research indicating an increased risk of spontaneous abortion among exercising women, provided the rationale for this thesis. Hence, the aim of the thesis is to examine whether current guidelines are scientifically well founded when it comes to the health of the child. The thesis thus illuminates possible consequences for the foetus of maternal exercise during pregnancy. All four studies in the thesis are based on data on about 90,000 pregnancies in the Danish National Birth Cohort, where detailed information on exercise are available from two different points of time during pregnancy.
About one third of the women were engaged in exercise during pregnancy, a little more in early pregnancy and a little less in late pregnancy. Most women were engaged in swimming, bicycling, or other low impact activities. Higher age, being a student, having an eating disorder, higher alcohol intake, and a health conscious diet were predictors for regular exercise, i.e. at least three exercise sessions per week. Holding a lower professional or a skilled worker job, multiparity, lower self-rated health, fertility treatment, smoking, a lesser health conscious diet, and a body mass index outside the normal range were predictors for not being engaged in regular exercise. Exercising women were slightly less likely to give birth before term than non-exercisers. This relationship was not affected by the type of exercise and did not affect the seriousness of the preterm birth. The exercise level of the mother was not associated with the newborn’s weight, length, ponderal index (corresponding to body mass index in newborns), head and abdominal circumference, or the weight of the placenta. Yet, physically active women had a slightly lower risk of having a small- or large-for-gestational-age baby, i.e. in the lower and upper 10th percentiles of the weight distribution for a given gestational age at birth. In a comparison between pregnant swimmers and bicyclists, based on the hypothesis that exposure to chemically purified water is associated with negative reproductive outcomes, there were no substantial differences in the average size of the babies at birth or in the risk of giving birth preterm, having a prolonged pregnancy or having a baby with a congenital malformation.
Results from this thesis do not contradict the recommendations that pregnant women should be physically active, since none of our findings suggest adverse health outcomes related to exercise during pregnancy.