So many theories have been proposed on the cause of morning sickness that it's sometimes called the "disease of theories," says Dr. Jeffrey Ecker, a San Diego obstetrician.
Some of the widely circulated myths suggest that morning sickness is more common with female fetuses or that it's related to which ovary (the right is blamed) releases the egg that becomes fertilized.
Neither is true.
What is true is that women carrying twins or multiple fetuses, those in first pregnancies, and younger women tend to have a higher incidence of morning sickness. But, Ecker says, "I think you have to view this with a grain of salt."
Besides the popular--but false--belief that morning sickness is psychological, some experts have proposed thyroid malfunction as a cause. But this has not been proven, Ecker says. Changes in the gastrointestinal tract that cause digestive processes to slow to a crawl during pregnancy have also been suggested but remain unproven.
Zinc, copper and Vitamin B6 deficiencies have also been blamed. But, says Ecker: "There is no reason to think that Vitamin B6 would be deficient in 70% of women"--the proportion of pregnant women who have nausea.
Ecker and other experts believe the hormone estrogen may be the most likely cause. But studies attempting to show that nausea and vomiting occur when estrogen levels are extremely high have been unconvincing, Ecker notes. "The hormone levels just don't correlate well with the symptoms."