What it does: Progesterone is made early in pregnancy by a cyst on the ovary called the corpus luteum. The corpus luteum continues to produce progesterone until about 10 weeks, when its production is taken over by the placenta. In the first trimester, levels of progesterone rise exponentially, and then they plateau. Progesterone does some very important jobs along the way: It keeps the uterus muscle relaxed and plays a role in the immune system helping the body tolerate foreign DNA (that is, the fetus).
The down side: Progesterone relaxes all smooth muscle (most important, the muscle wall of the uterus or "womb") in the body. It also leads to relaxation of the blood vessels throughout the body, prompting lower than normal blood pressure and occasionally dizziness, as well as all the not-so-fun gastrointestinal symptoms of pregnancy that include heartburn, reflux, belching, nausea, vomiting, gas, and constipation. Progesterone can also increase hair growth -- you may notice unwanted hair on your breasts and lower abdomen, for example.