by: Donna Tasco Poplawski, RN, C, MSN, WHNP
Pregnancy is an amazing phenomenon of nature that is the core of our existence. Yet even today, few people really have an understanding of what goes on inside a pregnant woman. It's not just nine months of belly growth followed by the birth of a baby. There is a partnership of "players" inside the mother's body that are the true "developers" of her baby. These partners -- the uterus, placenta, umbilical cord, and amniotic fluid -- are the key players which help the baby grow and develop until a time that is safe for him or her to be born.
The baby -- or "fetus" as it is known while still inside the mother -- is actually a parasite. The fetus physically cannot exist without the mother at this time and needs certain things from her to grow and develop to a stage that he can survive on his own. From his mother and through this partnership of players the fetus receives nutrients and oxygen, and eliminates his waste products through blood that is exchanged between himself and his mother. And it is the "players" who make this possible. Without them, there would be no vehicle for this crucial exchange between mother and fetus.
The uterus, when not pregnant, or gravid, is a pear shaped organ approximately the size of a fist. It has thick muscular walls that have the ability to stretch to several times its normal size along with the capability to house a fetus (or fetuses), a placenta (or placentas; see below), and a corresponding amount of amniotic fluid. It continues to stretch and change shape to accommodate its growing contents until the end of the pregnancy.
It begins the journey of pregnancy as a thick muscular structure resembling and upside-down pear. As it continues growing it changes to a rounder shape in the middle of the pregnancy, or ‘second trimester’. Moving into the "third trimester," or towards the end of the pregnancy, the uterus takes on a more oval shape and the walls are now very thin to better accommodate the growing fetus, placenta, and increase in amniotic fluid. This thinning of the walls also makes the muscle fibers stronger to more effectively push the baby out during labor and delivery.
Attached to the bottom of the uterus is the cervix. This is the neck-like part of the upside-down pear shaped structure that shrinks in length and dilates (opens) to allow the baby to pass from the uterus and into the birth canal (vagina) during birth. The uterus does the work and the cervix makes the changes during labor and delivery. All this activity causes the pain of labor.
By about six weeks after birth, the uterus, cervix, and vagina are back to their normal shape and size. However, they never quite go back to their original tight shape.
The placenta is amazing! It's my favorite player. Its function is fascinating as it is the link between mother and baby. This is where the exchange of nutrients, oxygen, and fetal waste products occurs. It has two sides: the maternal side, which is the side attached to the mother's uterine wall; and the fetal side, which is the side next to the fetus.
The maternal side is very dark red, spongy, and lobular (bumpy). It looks like a piece of raw liver. This is the busy side of the placenta where the exchange really takes place. This side has little finger like projections (villi) that imbed into the uterine wall so that the mother's blood and the fetus' blood can pass back and forth the necessary nutrients, oxygen, and fetal waste products that help maintain a healthy pregnancy.
The fetal side is smooth and shiny because a thin membrane called the amniotic membrane covers it. It has large blood vessels that start in the umbilical cord and fan out over the placenta. The amniotic membrane also functions as the amniotic sac. This is what encapsulates the fetus and amniotic fluid throughout the pregnancy. The membrane continues growing throughout the pregnancy to accommodate the enlarging fetus, placenta, and increasing amount of amniotic fluid.
Once the baby is delivered and the cord is cut, the placenta knows its job is done and usually within less than five minutes the placenta peels off from the uterine wall and is expelled through the vagina. Its life is over, even though the baby's is just beginning. It's an amazing process!