First Trimester

The first trimester includes the pre-embryonic period and encompasses weeks two through ten. You might wonder what happened to weeks one and two. Pregnancies are conventionally dated with reference to the woman's last menstrual period. That's because most women don't know when conception actually occurred, but they can usually remember the first day of their last period.

The due date is calculated by adding 280 days (40 weeks) to the date of the first day of the last menstrual period. Obstetricians and midwives don't actually do the addition, of course. They use a pregnancy wheel, a plastic or cardboard wheel that is designed to show the date 40 weeks from any date on the calendar. The due date is not absolute: Only 16 percent of babies are actually born on the due date. The vast majority of babies is born between 38 and 42 weeks.

During the first trimester, most of the action takes place on the inside, and what a lot of action there is. You don't automatically look pregnant, and initially you might not even feel pregnant. But you are very likely to develop a lot of symptoms that are the result of the pregnancy. That's because the developing placenta produces hormones that have a variety of effects on maternal organs. For example, during the first trimester, the placenta produces hormones that already begin acting on the breasts in preparation for breast-feeding. Increased breast growth occurs that can cause temporary breast tenderness. And everyone knows about pregnancy-induced nausea ("morning sickness"), but tiredness and weight gain are also very common. You may even notice that your hair looks better when you're pregnant. Because of the increased blood flow associated with pregnancy, hair grows faster and thicker. Unfortunately, after delivery things return to normal.

The baby undergoes its most complex and important stage of growth during the first trimester. In fact, a human being grows faster during this period than at any other time in pregnancy, childhood, or puberty. The fertilized egg divides repeatedly, and the resulting cells organize into the embryo and the placenta. By week 12, all external structures and internal organs have been formed. The embryo is easily recognizable as human and has begun moving freely within the amniotic sac. Over the next 28 weeks the fetus grows and the organ systems mature, but no new structures develop.