A Staff Report from the Straight Dope Science Advisory Board

Why do pregnant women get morning sickness?

November 8, 2001

Dear Straight Dope:

After spending the better part of this morning searching your archives for this topic to no avail (interrupted frequently by trips to the bathroom), I find myself turning to you for the answer to the following: WHY DOES PREGNANCY CAUSE MORNING SICKNESS??? My doctor isn't particularly sympathetic--he tells me this is normal and to eat crackers. Duh. I'm about 12 weeks along and spend more time staring at the plumbing of my house than I do pondering baby names. I understand this is something all women deal with, but I just want to know WHY, and if you would kindly find the answer for me, I will name the child (male or female) after you, solving two problems at once.

Um, if I said, "no one really knows," am I still eligible in the baby naming?

The problem is, we really don't know. Morning sickness (also called "nausea and vomiting of pregnancy," or NVP, since it doesn't always strike in the morning) affects between 50 and 85 percent of all new mothers.

Some of the suggested reasons:

  • An increase in the hormone progesterone relaxes the uterine muscles, which prevents early labor... but may also relax the stomach and intestines, leading to excess stomach acids.
  • A buildup in hCG (human chorionic gonadotropin) may be the culprit, in which case things will settle back to normal after the 12th week of pregnancy and hCG levels decrease.
  • An increase in sensitivity to odors, meaning that you're smelling some new scents you never could before, and they ain't pretty.
  • Eating vegetables. Margie Profet theorized in her book Pregnancy Sickness: Using Your Body's Natural Defenses to Protect Your Baby-To-Be (1997) that the small amounts of toxins vegetables produce to fight off insects are normally harmless to humans but extremely dangerous to embryos; therefore, becoming nauseous during pregnancy was an evolutionary step towards protecting the fetus. Eat bland foods and you eat less toxins; thus, women suffering from NVP were less likely have children with birth defects. On the other hand, Judith Brown, a nutritionist at the University of Minnesota's School of Public Health, has linked consumption of fruits and vegetables to higher birth weights, and higher birth weights tend to mean healthier babies. So there's no clear consensus on whether eating vegetables is good or bad.

So, unfortunately, there it is. We don't know what causes NVP, though we have quite a few theories. We do know that for half of all sufferers, it ends by the 16th week of pregnancy. On the bright side, studies have shown that women who suffer from NVP are less likely to have miscarriages. All we can suggest is to consult with your physician and see what sorts of anti-nausea treatments you can take. Maybe some Velveeta on those crackers?

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