Nobody knows why morning sickness occurs.
Some researchers believe that eons ago it was the body's way of keeping the mother away from potentially toxic foods that could harm the developing embryo. Nowadays, many pregnant women are turned off by coffee, alcohol and cigarette smoke, things that may indeed hurt a developing baby. In general, morning sickness doesn't endanger the fetus. An embryo needs just a few calories to keep growing, so don't worry if you're able to eat only a little because of nausea and vomiting. It is important to keep taking your prenatal vitamins as much as you can, because supplements like folic acid are necessary to the developing embryo.