An intrauterine pregnancy is a pregnancy that happens inside of the womb. The fertilized egg implants itself on the interior wall of the uterus. This condition may be referred to as a normal pregnancy. If the egg implants itself anywhere outside of the womb, as in the fallopian tube or elsewhere, it is called an ectopic pregnancy. The gestation of the fetus in an intrauterine pregnancy typically takes from 38 to 42 weeks, with an average of 40 weeks.
Although pregnancy is different from one woman to the next, the first signs of pregnancy may include a missed menstrual period, breast tenderness, nausea, vomiting, or fatigue. An ultrasound test usually can confirm an intrauterine pregnancy and determine how far along the pregnancy has progressed. The pregnancy is divided into three trimesters — the first is from conception to 12 weeks, the second from 13 to 28 weeks, and the third from 29 weeks until birth.
After the fertilized egg is implanted in the uterine wall, a placenta develops from the endometrium — the mucous membrane lining the uterus. This placental sac is attached to the embryo by an umbilical cord, brings nourishment from the mother, and carries away waste products from the embryo. Once the second trimester is reached, the embryo is typically referred to as a fetus.
Throughout an intrauterine pregnancy, a woman’s body goes through many changes. For instance, hormonal production and balances shift — typically, more estrogen is produced during one pregnancy than during a woman’s entire non-pregnant life. During the second trimester, the mother’s heart needs to pump more blood to provide for the fetus. As a result, the mother’s heart works 30 to 50 percent harder, and by the end of the pregnancy, the fetus is receiving about one-fifth of the total blood supply. Often, a woman’s internal organs shift to accommodate the ever-growing uterus as it takes up more space within the body cavity.
Each change in the mother and fetus combine to prepare them for the eventual birthing process. During a vaginal birth, a woman’s cervix, or entrance to the birth canal, enlarges. The fetus typically changes position, with the head toward the birth canal, and the uterine muscle contracts and relaxes, thereby pushing the baby toward the canal and out. When a Cesarean delivery is performed, an incision is made into the woman’s abdomen and the baby is lifted out of the body. In both cases, the umbilical cord is cut and the placenta is removed from the mother.