Evolutionary biologists have suggested that morning sickness is a mechanism to help women avoid noxious substances that might harm the fetus at an early stage of development.
Formerly, scientists had assumed that the sickness early in pregnancy was related to a sharp rise in the hormone estrogen.
But as some scientists point out, the sickness almost always occurs in the first three months of pregnancy, when organs are in a formative stage and therefore most vulnerable to trace poisons in the diet.
Nausea would help a woman temporarily avoid foods that contain substances that are harmless to adults but that might cause birth defects or miscarriage.
Studies have found that there is a higher rate of miscarriage among women who have little or no morning sickness.