The female reproductive system is studied in the medical specialty of gynecology. Gestation, usually lasting nine months or forty weeks, is divided into three trimesters and refers to the period from conception to birth. In the first trimester from weeks 1-12, the embryo develops into a fetus and all major organs and body systems are formed. The second trimester from weeks 13-27 is usually the easiest stage of pregnancy, though morning sickness may subside. In the third and final trimester from week 28 until birth, the baby grows and matures while fetal movement increases as discomfort rises in preparation for delivery.