Vomiting during morning sickness reduces the likelihood of miscarriage, pre-term birth, low birth weight and death of the newborn. The theory holds that vomiting makes pregnant women avoid foods that have chemicals likely to damage the developing central nervous system of the embryo and fetus.
From an evolutionary biological viewpoint loss of kilojoules due to food lost through vomiting is balanced by improved prevention of harm to the developing fetus.
Morning sickness usually ceases at the end of the first 3 months of pregnancy because by then the central nervous system of the fetus is less susceptible to the assault that a hazardous chemical could provoke.
Statistically, between 6 and 18 weeks into pregnancy, women who experience morning sickness have fewer miscarriages than women who do not experience it. And women who vomit during morning sickness have even fewer miscarriages than those who are merely nauseous. So if you are a woman with morning sickness, it might make you feel a little better to know that it is good for your baby.
A study of 26 women with severe morning sickness found that anti-vomiting medications (antiemetics) taken before symptoms of morning sickness started appeared to prevent recurrence of severe morning sickness in subsequent pregnancies. Another study found that the more pregnancies a woman experiences, the more likely she will experience morning sickness—a fetal ‘dose’ effect.