After implantation, the embryo begins the process of forming a placenta, a large organ made up of both embryonic and maternal tissue that will provide oxygen and nutrients to the developing baby through the umbilical cord. During this time, the level of hormones produced by the embryo increase and can cause some of the main symptoms associated with early pregnancy such as fatigue and “morning sickness” which is actually a misnomer because the nausea and vomiting it refers to can occur at any time during the day.

The first third of a pregnancy called the first trimester is the most tenuous time for a pregnancy. An estimated 25-40% of pregnancies are spontaneously ended in the first trimester, although many of these occur before the woman even knows she is pregnant. Most of these terminated pregnancies occur because something goes wrong in the early stages of development and the embryo is no longer viable. Still it is a good idea to be extra careful about what you eat and drink and what medications you take during this sensitive period.

In addition to the development of the placenta, other key developmental events to occur in the first trimester include: the heart begins to beat, the primitive brain and spinal cord (called the neural tube) form, the fingers and toes form followed by fingernails and toenails, and it begins to move.

The second trimester starts in the 14^th week and the fetus (no longer called an embryo) has reached 13 weeks gestational age. Before this point, many women and couples choose to keep their pregnancy a secret or to tell only close friends or family. Because this transition marks an important milestone and a significant reduction in the probability of miscarriage, this is often the time that people choose to begin telling everyone about the pregnancy. The morning sickness and fatigue of the first trimester usually resolve at the start of the second trimester.

Around weeks 18-20, an ultrasound may be performed to check on the growth and development of the fetus. At this time, the sex of the baby can also be determined if the parents wish to know. Other things that occur during the second trimester include the appearance of skin followed by a coating of fine hair called laguno, hearing ability begins, and taste buds develop, as do the sense of balance and the blood system. Movement increases dramatically and may be felt by the mother sometime between weeks 20-28.

The third trimester begins in the 28^th week of gestation. At the start of the third trimester, the baby’s eyes open. Due to the large weight gain and increase in size that occur in the third trimester, this period may mark a return to fatigue for the mother like that seen in the first trimester, along with increasing difficulty sleeping and even getting comfortable while awake. For the first 10 weeks of the third trimester, the fetus will grow half a pound a week. This rapid growth is accompanied by the continued development of reproductive, respiratory, and nervous systems. In the 32^nd week, the lanugo falls off, followed by the buildup of a pasty white coating that protects your baby’s skin called vernix.

At the end of the 37^th week, the fetus is considered full term, although many pregnancies continue until the official due date at 40 weeks or beyond. Doctors now induce labor (or deliver by cesarean section) two weeks past the due date because waiting any longer puts the mother and the fetus at risk.