What Causes Morning Sickness?

Buckingham Palace has confirmed what the British tabloids have suspected for a while: The Duchess of Cambridge is expecting. Unfortunately, Kate has also been admitted to the hospital due to hyperemesis gravidarum, or acute morning sickness—so severe that no food or liquid can be kept down. This is much different than the morning sickness that affects 50 to 80 percent of pregnant women, starting around the sixth week of their pregnancy. What is it about pregnancy that gives women the constant urge to purge?

It seems contradictory—in the early stages of pregnancy, when a mother needs to nourish her growing fetus, her stomach refuses to keep anything down. And many pregnant women will state categorically that “morning sickness” is a misnomer; the nausea likely lasts all day and night.

For many years, doctors attributed morning sickness to the plethora of hormones that race through the body during early pregnancy. These hormones heighten the sense of smell, making a woman far more sensitive to aromas than she was prior to pregnancy.

New Theories

However, in the late 1970s, scientists—including Ernest Hook, an endocrinologist at Albany Medical College—began floating a new theory: That morning sickness protected the fetus from toxins that could hinder or derail its development (the period between six and 18 weeks is crucial in a fetus's development). And in 1992, Margaret Profet argued that morning sickness was part of an evolutionary adaptation. Potential toxins nauseated the mother, so she avoided them and didn’t hurt her baby—or herself. (A woman's immune system is compromised during pregnancy.)

In 2000, researchers at Cornell University found evidence that supported this theory. They studied 79,000 pregnancies in 16 countries, and found that 65 percent of the women had an aversion to at least one food. Twenty-eight percent couldn’t tolerate animal products (meat, eggs, fish), 16 percent avoided caffeinated drinks, and eight percent disdained strong-flavored vegetables such as broccoli. All of the aforementioned items contain secondary compounds which are natural toxins. "Our study ... shows that nausea and vomiting in pregnancy is beneficial by expelling such foods as meat and strong-tasting vegetables that historically and still may contain harmful toxins and microorganisms that could potentially sicken the woman and damage her fetus just when its organs are developing and are most vulnerable to chemicals," said Paul Sherman, Cornell professor of neurobiology and behavior. The study also found out that women in societies that eschewed animal products and subsisted mainly on corn, rice, and tubers rarely suffered from morning sickness.

There are some scientists who disagree with this idea, though, because, among other things, sickness during pregnancy doesn't necessarily equal a better pregnancy outcome (i.e., healthier babies).

There is another theory that gained some attention earlier this year: SUNY-Albany psychologist Gordon Gallup believes that the cause of morning sickness is actually unfamiliar semen. Because half of the fetus's DNA comes from the father, he says the mother's body treats it as an infection, which triggers nausea and vomiting.