placenta fenestrata
placenta
[plah-sen´tah] (pl. placentas, placen´tae) (L.)In anatomic nomenclature the placenta consists of a uterine and a fetal portion. The chorion, the superficial or fetal portion, is surfaced by a smooth, shining membrane continuous with the sheath of the umbilical cord (amnion). The deep, or uterine, portion is divided by deep sulci into lobes of irregular outline and extent (the cotyledons). Over the maternal surface of the placenta is stretched a delicate, transparent membrane of fetal origin. Around the periphery of the placenta is a large vein (the marginal sinus), which returns a part of the maternal blood from the organ.
The major function of the placenta is to allow diffusion of nutrients from the mother's blood into the fetus's blood and diffusion of waste products from the fetus back to the mother. This two-way exchange takes place across the placental membrane, which is semipermeable; that is, it acts as a selective filter, allowing some materials to pass through and holding back others.
In the early months of pregnancy the placenta acts as a nutrient storehouse and helps to process some of the food substances that nourish the fetus. Later, as the fetus grows and develops, these metabolic functions of the placenta are gradually taken on by the fetal liver.
The placenta secretes both estrogens and progesterone. After birth of the infant the placenta is cast off from the uterus and expelled via the birth canal.
With the onset of any contractions and cervical dilation, or when the cervix begins to dilate at the onset of labor and the upper and lower uterine segments differentiate, the placenta is stretched and pulled from the uterine wall, producing bleeding. The bleeding usually is abrupt and painless and may stop on its own. However, if it continues it can be life-threatening for the mother since it is maternal blood that is being lost. The life of the fetus is in jeopardy because of anoxia resulting from separation of the placenta from its blood supply.
Diagnosis can be established by ultrasonography or radiologic placentography. Once diagnosis is made, treatment will depend on the gestational age of the fetus and the percentage of placenta covering the cervical os. Cesarean delivery is recommended if 30 per cent or more of the opening is obstructed by the placenta. If there is minimal bleeding that stops on its own, the fetus is not in distress, and if the gestational age is such that continuing the pregnancy is necessary for delivery of a viable fetus, the pregnancy may be continued under careful monitoring in the hospital, or at home if the mother is able to stay in bed. However, if the life of the mother or fetus is threatened by continued and excessive bleeding, delivery is indicated.
Vaginal examinations are carried out in an operating room so that if hemorrhage does occur as a result of manipulation of the uterus, a cesarean section can be done immediately to remove the placenta, stop the bleeding, and deliver the child safely.
Postpartal hemorrhage and infection are more likely in women who have had placenta previa. Placement of the placenta in the lower segment predisposes to more bleeding because that portion of the uterus does not contract as forcefully as the upper segment. Additionally, the misplaced placenta has enlarged its bed to compensate for its poor location, so that there is a larger denuded area after delivery of the placenta. The same denuded area is also more susceptible to infection because it is located near the cervical opening where infectious organisms may enter.
Vaginal bleeding during pregnancy or labor is frightening for the mother. She will need reassurance and frequent explanations of what is happening to her throughout the period of monitoring and delivery. Some emotional stress can be alleviated by encouraging the mother to be aware of fetal movements and allowing her to listen to normal fetal heart sounds.