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If you’ve ever been pregnant, you know that the agony that is morning sickness -- and it’s not just something that happens in the morning! SciShow explains the many theories about what causes it.
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Morning sickness, also known as nausea gravidarum, is that nauseous, fatigued feeling that has been ruining days of pregnant women for all time. The term "morning sickness" is kind of a cruel joke, since, if you've ever been around a pregnant lady or been a pregnant lady, you know that the agony can strike any time of day or night or just linger. All the time. It's not fun.

This nausea can be mild--similar to feeling sleep deprived, hungover, or like you were stuck on the tilt-a-whirl all day--or it can be strong enough to cause vomiting. Which I probably don't have to tell you is far less cute and funny as it's often portrayed in romcoms where the heroine daintily pukes into a wastebasket in front of her coworkers and then suddenly feels fresh as a daisy.

For a small proportion of women, around one percent, these symptoms become so severe that they may lead to weight loss, dehydration, a dangerous drop in blood acidity called alkalosis, and hypokalemia or low blood potassium levels. This extreme morning sickness is called hyperemesis gravidarum, and it can be very dangerous. Luckily, no matter how bad you may get it, most women find their morning sickness starts to fade in the second trimester.

So, what causes this misery? Why wasn't nature satisfied with pregnancy causing just bloating and backaches and constipation and weird facial skin discoloration?

Well, the short answer is that we... don't really know, but because this is science we're talking about, there are, of course, several theories.

Many experts believe that those craptastic feelings are triggered by the truly insane amount of hormonal changes pregnant women experience early on. Especially the increase of human chorionic gonadotropin, or HCG. HCG is produced by the developing embryo after conception and later by the placenta. It's released to help the ovaries keep producing the hormones necessary to sustain pregnancy.

In most normal pregnancies, the level of HCG in a woman's body doubles every two or three days, getting higher and higher until it peaks around the third month. Some believe these soaring hormone levels overstimulate the chemoreceptor trigger zone, or CTZ, the part of the brain that controls the so-called "vomit reflex." And then, it's hello, Puke City.

But HCG is only one of several tricky hormones at work here. Some scientists suggest estrogen levels may be partly to blame because they can be one hundred times higher during pregnancy than they normally are. Although, so far, studies have found no correlation between estrogen levels in pregnant women who experience morning sickness and those who don't.

And then there's progesterone, which also skyrockets during pregnancy in part to relax the uterine muscles to prevent early childbirth, but this hormone also relaxes the stomach and intestinal muscles, which may lead to extra stomach acid and acid reflux, which could factor into morning sickness.

Hypoglycemia or low blood sugar caused by the energy drained from the growing placenta may also play a role in the stomach upset, which is why doctors recommend pregnant women eat lots of small meals throughout the day. Still others maintain the heightened sense of smell associated with pregnancy may similarly make some mamas extra sensitive to unpleasant smells.

Perhaps the most interesting theory, though, and it is just a theory, suggests that morning sickness may actually be a useful evolutionary adaptation. The idea here is that easily triggered nausea may help protect expectant mothers from eating the wrong thing and getting food poisoning, thus protecting their babies from toxins.

When you're feeling crappy, you tend to look for foods that are naturally low risk for contamination. Simple carbs, like crackers and rice and bread, tend to be far more appealing to a queasy mom than meat and eggs and dairy, even certain vegetables, all of which can spoil easily in ways that can be hard to detect. So perhaps the body is telling the mother to stick to safer, blander foods to increase her child's chances for survival. And it so happens that a developing baby's vulnerability to certain toxins peaks at the end of the first trimester, which is also when morning sickness tends to ease up.

We may never know the exact causes of morning sickness, but the results are often painful and plain, so make sure you're extra nice to all those pregnant ladies. They're traveling a rough road.

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