Pregnancy Info states that the missionary position may become increasingly awkward for pregnant women as their belly begins to grow. The March of Dimes notes, "Positions that work before pregnancy and early in pregnancy can be uncomfortable or even unsafe at later stages of the baby's development. For example, a woman should avoid lying flat on her back after the fourth month of pregnancy, because the weight of the growing uterus puts pressure on major blood vessels." David Port states, "Beginning early in the second trimester, doctors tend to discourage pregnant women from supine exercise. And the missionary position is exactly that kind of exercise, at least if the activity lasts more than a few fleeting moments."