Exercise during pregnancy and the postpartum period. ACOG Technical Bulletin Number 189--February 1994

In the absence of either obstetric or medical complications, pregnant women can continue to exercise and derive related benefits. Women who have achieved cardiovascular fitness prior to pregnancy should be able to safely maintain that level of fitness throughout pregnancy and the postpartum period. Depending on the individual's needs and the physiologic changes associated with pregnancy, women may have to modify their specific exercise regimens. Despite findings that suggest lower birth weights among offspring of women who continue to exercise vigorously throughout pregnancy, there currently are no data to confirm that, with the specific exceptions mentioned here, exercise during pregnancy has any deleterious effects on the fetus. While maternal fitness and sense of well-being may be enhanced by exercise, no level of exercise during pregnancy has been conclusively demonstrated to be beneficial in improving perinatal outcome.

Published by International journal of gynaecology and obstetrics: the official organ of the International Federation of Gynaecology and Obstetrics, 1994. The guideline itself is at the source linked here.

pubmed.ncbi.nlm.nih.gov/7913067/