Expectant mothers are virtual magnets for unsolicited advice. Mothers, grandmothers, aunts, sisters, sisters-in-law, new mothers, friends, and even strangers offer what seems to be an endless supply of supposedly authoritative opinions on every aspect of pregnancy: A craving for spicy food denotes a boy. Carrying the baby low denotes a girl. Besides gender predictions, a pregnant woman is also apt to acquire an earful of advice about miscarriage,...