Pregnancy Trimesters

The following is an overview of the development process during the three pregnancy trimesters. The two methods used to measure the duration of pregnancy are gestational age and fertilization age. Gestational age is measured from the first day of your last menstrual period, which is about 2 weeks before actual fertilization. Usually, obstetricians or family physicians calculate the duration of your pregnancy on the basis of gestational age, so this section will also.

The First Trimester:

The first 3 months of fetal development are in many ways the most important. During this time, all the major organs in the body are formed. By the end of this period your baby is not more than 3 inches long and weighs little more than l ounce. The time from fertilization of the sperm and egg to implantation in the uterus is about 5 to 7 days. After burrowing deep within the uterus, the egg begins to grow, doubling in size every day. By now, the placenta has begun to form. In another week, the rudiments of a spinal cord are evident and within days, five to eight vertebrae are in place. In addition, the eyes and heart hate begun to form.

It is during the third week after fertilization that the embryonic period begins. Before this, the products of conception are referred to as an ovum. This is about the time that an expected period does not happen. If you have a pregnancy test, the results probably will be positive. Over the next few weeks the components of a human being develop, although at first the human baby is similar in appearance to the developing babies of some other mammals. The head begins to form, as does the intestinal tract. At the end of the sixth week the brain becomes more noticeable, and arm and leg buds begin to appear. Cells that will later become either an ovary or testes have appeared.

By the seventh week, the chest and abdomen are fully formed and the lungs are beginning to develop. The embryo measures slightly more than one-half inch and weighs a fraction of an ounce. Your baby’s face and features are forming in the eighth gestational week. Fingers and toes are beginning to develop, as is the ovary or testis. If the embryo is a male, his penis begins to appear at this time. At the end of the second month of pregnancy, your baby looks like a human infant, albeit in miniature. By the 10th week, your baby's face, with the exception of the jaws, is very developed. The heart has four chambers and beats at 120 to 160 beats per minute. At this point, the embryo is considered a fetus. By the end of the first trimester, the fetus has a head that is disproportionately large compared with the rest of its body.

The Second Trimester:

During the second trimester the fetus grows and the organs that formed during the previous weeks mature. At 13.5 weeks the fetus has tiny finger nails. The genitals are fully formed, and the sex can be determined with certain prenatal tests. The fetus can kick and move its toes. The mouth can open and close, and the fetus is capable of bending its arms and of making a fist. By the end of the fourth month, the heart beat can be detected with a stethoscope (other specialized instruments can detect a heartbeat much earlier). You are also likely to feel the first signs of life in your abdomen. The fetus’s skin at this point is slightly pink and less transparent than it was previously. Fine hair covers the entire body. The first eyelashes and eyebrows begin to appear. One month later, the fetus may have hair on its head. Fat deposits begin to appear beneath the wrinkly skin. The fetus is now 12 inches long and weighs about 1 pound. If it is born at this time, it will attempt to breathe but probably will not be able to survive.

The Third Trimester:

The fetus takes on most of its weight during its last 13 weeks of development. At the beginning of this last phase of development the fetus weighs slightly more than a pound. The average baby is born 3 months later weighing 7.5 pounds. When you are 28 weeks pregnant, your baby is covered with a thick white protective coating called vernix. The infant’s eyes are open, and a baby born at this time can cry weakly and move its limbs. Although an infant at this stage weighs only 2 pounds, two out of three babies born at this stage survive because of recent advances in the care of premature and ill newborns, although they may experience significant medical problems.

One month later the male infant’s testicles descend into the scrotum. The infant now weighs 3 pounds 12 ounces. The vast majority of these infants are also capable of surviving with proper care. The infant born at term or at 40 weeks after the mother’s last menstrual period has a more rounded body and is less wrinkly than babies born earlier. The skin may or may not still be covered with vernix. Most of the body hair is gone, although the shoulders and arms may still have a light covering. The fingernails and toenails may extend beyond the fingers and toes.