Morning sickness has become synonymous with pregnancy in our culture. But certainly not every pregnant woman experiences it. (A medical term used to refer to extreme forms is hyperemesis gravidarum.) Only a little more than half of pregnant women are afflicted with nausea.

Many people believe that pregnancy sickness is the body’s natural defense system at work, protecting your baby from harmful toxins. It’s easy to jump to the conclusion, then, that if you’re not sick, your body must not be protecting your child from harmful toxins. But it doesn’t work that way. The bottom line is that no conclusive evidence exists that not being sick is a bad sign.

Nausea is caused by a number of factors. The most popular theory about morning sickness is that it’s due to elevated hormones, primarily human chorionic gonadotropin and estrogen. Researchers believe nausea may have to do with the amount of hormones circulating and perhaps the structure of certain hormones or your particular sensitivity to them. Also, if you have a pre-pregnancy tendency toward motion sickness, it may mean the area of your brain that controls nausea and vomiting is more sensitive.

Environment can also play a role. Smells such as perfume, dog food, or coffee, as well as motion and the sight of certain foods (a friend of mine would almost instantly become nauseous at the sight of a can of tuna) can all trigger nausea. Also, stress, fatigue, and operating on an empty stomach can make you more prone to gastrointestinal upsets.

The bottom line is, nobody really knows why some women get sick and others don’t. What is important to know is that whether or not you experience morning sickness has nothing to do with how successful your pregnancy will be.