Inflammation of the placenta is usually secondary to infection of the membranes. Most often such infections follow the introduction of pus-forming bacteria into the uterus by instrumentation through the vagina; they are the aftermath of prolonged labour or of prolonged rupture of the membranes. If labour is prolonged, bacteria penetrate the fetal side of the placenta, enter the fetal circulation, and often cause death of the infant after delivery.

The placenta may become infected from organisms in the maternal blood. Maternal syphilis, toxoplasmosis, tuberculosis, and malaria may affect the placenta. The viruses of chickenpox and smallpox may cause placental lesions. A number of pathogenic bacteria and viruses cross the placenta and sometimes kill the fetus without causing any specific changes that have been noted in the placenta.

Placental anomalies

Abnormalities in the structure of the placenta are relatively common. It may be partially divided into two or more lobes; there may be extra lobes; or the placenta may be divided into two or more separate structures. Abnormal placentas result from shallow and from deep implantation. The former type, called placenta circumvallata, is associated with several maternal and fetal complications; the latter type, called placenta membranacea, may cause problems at delivery—e.g., bleeding, failure of the membrane to separate.

Hydramnios

Hydramnios, sometimes called polyhydramnios, is the presence of an excessive amount of amniotic fluid. Normally the uterus contains approximately 1,000 millilitres (slightly more than one quart) of amniotic fluid; anything over 2,000 millilitres is abnormal. Accumulations of more than 3,000 millilitres occur in approximately one pregnancy in a thousand. Lesser degrees of hydramnios probably occur in about 1 in 150 deliveries. The appearance of large amounts of fluid within the space of a few days is rare; such a condition is met with in fewer than 1 in 4,000.

Hydramnios occurs most often in association with fetal abnormalities, particularly those of the nervous, digestive, and renal systems; when the fetus has erythroblastosis, a disease resulting from incompatibility between the infant’s and the mother’s blood; when there is more than one fetus; or when the mother has diabetes or preeclampsia. Almost all pregnancies in which the fetus suffers from obstruction of the esophagus and half of those in which there are severe brain anomalies are accompanied by excessive amniotic fluid.

Acute hydramnios causes rapid overdistention and enlargement of the uterus. The woman experiences abdominal pain, nausea and vomiting, and difficulty in breathing. Her heart and blood vessels are put under severe stress; she may show signs of heart failure. Swelling of the feet and legs develops. These manifestations are all caused by the pressure of the rapidly enlarging uterus upon the other viscera.

Chronic hydramnios usually causes enough pressure from the abnormally enlarged uterus to make the affected person uncomfortable.

The cause of hydramnios is unknown. The most tenable theory is that there is a reduction in the amount of fluid that passes from the fetus to the mother and an increase in the amount that passes from the fetus to the amniotic sac. This would explain the relationship between fetal anomalies and hydramnios.

Many pregnancies complicated by an abnormal amount of amniotic fluid terminate prematurely. The fetus has a greatly increased chance of suffering from congenital anomalies. Roughly half of the babies in this group have been lost in the series of cases that have been reported. The greater the amount of fluid, the higher the fetal mortality. Women with hydramnios also are faced with a somewhat higher risk. Premature separation of the placenta and postpartum hemorrhage are the two most significant maternal complications associated with it.

Minor degrees of hydramnios require no treatment. Removal of the excess fluid is the only effective management if symptoms from uterine distention become too distressing. This may be done either by perforating the membranes through the cervix or, preferably, by inserting a needle through the abdominal wall and the wall of the uterus; care is taken to avoid injury to the woman’s bowel or the placenta. Either procedure is likely to start labour.

Oligohydramnios

True oligohydramnios, a deficiency in amniotic fluid, is a rare condition of unknown cause. It is seen more often in pregnancies that have extended beyond the projected time of delivery. If it occurs early in pregnancy, there are usually firm adhesions between the membranes and the embryo, with distortion of the fetus. A decrease in the amount of fluid later in pregnancy allows the membranes and uterine wall to press on the baby. The baby’s position is distorted, and as a result it may be born with a clubfoot or wryneck. Its skin is dry and thickened. Defective development of the kidneys is common with oligohydramnios. As a rule, the condition causes the mother no distress, but the infant has a greatly increased chance of being born with major anomalies.