A: Since most pregnancy books advise women to avoid lying on their back throughout pregnancy, you can only imagine the frantic phone calls I receive when a patient discovers that she slept on her back.

After calming her down, I explain the facts surrounding this myth. When a pregnant woman lies on her back, the uterus may press on the large blood vessels carrying blood from her legs and pelvis, slowing its return to the heart. The fear is that this supposedly reduces the amount of blood available to the baby. Perhaps this is true in a rare case, but nobody has ever proven that it is dangerous to the baby.

I reassure my patients by telling them that the baby is perfectly safe and has not suffered any injury. I continue by explaining one of the great truths of pregnancy: God (or nature) was smart enough to create woman so she'd turn on her side, awake or asleep, to improve her circulation in this situation. If your partner is still worried, remind her that cavewomen managed to have babies without prepared childbirth classes or maternity books. Sometimes we have to trust our bodies to do the right thing.

To answer the question simply, your partner should sleep any way she feels comfortable. But she should avoid lying on her back while exercising; read more about this in my book, Pickles and Ice Cream: A Father's Guide to Pregnancy.