The placenta is an organ unique to pregnancy. Throughout pregnancy, it acts as a transport service between the mother and the baby. The placenta transfers oxygen and nutrients from the mother's bloodstream to the baby and carries fetal waste products in the opposite direction.
In the third trimester, the two main problems that can occur with the placenta are often signaled by the same symptom: vaginal bleeding. Any amount of bleeding in late pregnancy should be reported to your doctor immediately.
Causes and Risk Factors: Placental Abruption
Abruption of the placenta refers to the separation of the placenta from the inner wall of the uterus before labor begins. It can decrease or interrupt the flow of oxygen-rich blood to the baby. Placental abruption can cause fetal death in the third trimester. It can also cause the mother to go into shock as a result of hemorrhage or to have severe circulatory problems. Fortunately, with close monitoring of the mother and baby and prompt delivery at signs of trouble, the outlook for both is good.
Separation of the placenta may be partial, involving only a part of the placenta, such as an edge. Separation of the placenta may be complete, in which the entire placenta is separated from the inside of the uterus. Although placental abruption always causes some bleeding, the blood may not always be apparent. Sometimes the middle portion of the placenta pulls away from the uterine wall, leaving the outer margins and membranes attached. Blood can thus be trapped and concealed in a "pocket." At other times, the baby's head or another body part may be so tightly pressed against the wall of the uterus that the blood cannot make its way past.
Placental abruption’s cause is unknown, but it appears to be more common in black women, in women who are older (especially those older than 40), in women who have had many children, and in women who smoke. By far, however, the most common condition associated with placental abruption is hypertension in pregnancy. Women who have high blood pressure during their pregnancy — whether the condition first developed while they were pregnant or was present before — are more prone to placental abruption.
The risk of abruption also seems to be higher with premature rupture of the membranes. This is a condition in which the membranes that surround the fetus break too early in pregnancy, before labor begins. Very rarely, trauma or injury to the mother may cause placental abruption.
In the early stages of placental abruption, there may be no indication that it is happening. When symptoms do occur, the most common is bleeding from the vagina. The bleeding may be scant, heavy, or somewhere in between, but the amount does not necessarily correspond to how much of the placenta has separated from the inside of the uterus. Other symptoms that may be caused by placental abruption include back or abdominal pain, uterine tenderness, and rapid contractions.
Diagnosis of Placenta Abruption
When a woman has vaginal bleeding in the third trimester, her doctor will usually try to exclude causes such as placenta previa (see below). Placental abruption is diagnosed through a process of elimination of other possible causes of the bleeding. An ultrasound exam may be done to try to detect a separated placenta, but often the condition is not detectable by this technique.
Management of Placental Abruption
When placental abruption is suspected, the steps taken depend largely on the condition of both the baby and the mother. Electronic monitoring is usually used to look at patterns of the baby's heart rate. If monitoring shows no signs that the baby is in immediate trouble, the mother may be hospitalized so that her condition can be monitored closely. This may be the chosen course if the pregnancy has not yet reached term.
Signs that the baby is in jeopardy will prompt immediate delivery. If there is severe bleeding, the mother may need blood transfusions. Cesarean delivery may be necessary, although in some situations vaginal birth may be possible.
Outlook for the Future
Unfortunately, there is an increased risk (about one in 10) that placental abruption will recur in a woman's subsequent pregnancies. The good news is that, with close monitoring and prompt action at signs of danger to the baby, most of these mothers and babies get safely through birth with no long-term ill effects.
At term, the placenta normally is located high up near the top (fundus) of the uterus. But in some pregnancies, the placenta lies low in the uterus and may partly or completely cover the opening of the cervix. This condition, called placenta previa, poses a potential danger to mother and baby because of the risk of hemorrhage (excessive blood loss) before or during delivery.
Placenta previa may take one of several forms:
1. Marginal. The edge of the placenta is at the margin of the cervical opening. As the cervix dilates during labor, more of the placenta may move upward. Vaginal delivery may be possible under certain conditions.
2. Partial. The placenta partly covers the cervical opening. Vaginal delivery is likely to result in hemorrhage as the blood vessels in the placenta rupture during labor.
3. Total. The placenta completely covers the cervical opening, making vaginal delivery impossible because of the risk of massive bleeding.
Although the placenta may lie close to the cervical opening in the second or early third trimester, it almost always migrates up toward the top of the uterus as term approaches. This is referred to as low-lying placenta.
The cause of placenta previa is not known for certain. Like placental abruption, it is more common in women who have had children before, in older women, and in women who smoke. A previous cesarean birth or induced abortion also seems to increase the risk of placenta previa. And when there's a large placenta, the risk of placenta previa is increased because it is more likely for the edge of the placenta to lie near or over the cervical opening.
Placenta previa most often presents itself with painless vaginal bleeding. Most often this occurs near the end of the second trimester or the beginning of the third. The blood from placenta previa is usually bright red, and the amount may range from scant to heavy. The bleeding may stop on its own at some point after it starts, but it nearly always recurs days or weeks later.
If you notice bleeding in late pregnancy, don't assume it's harmless, even if it goes away on its own. Any bleeding in the third trimester should be reported to your doctor right away.
Diagnosis of Placenta Previa
An ultrasound exam is effective for detecting the location of the placenta. Up to 98 percent of cases of placenta previa may be detected in this way. A cervical exam, in which the entrance of the cervix is gently probed, is done only under certain circumstances when placenta previa is suspected. Because even the gentlest cervical exam can cause hemorrhage, it is done only when delivery is planned, and only when an immediate cesarean delivery can be performed.
Management of Placenta Previa
How placenta previa is managed depends on two factors:
1) whether the fetus is mature enough to be born and 2) whether there is active bleeding from the mother's vagina.
If the placenta is found to be close to, but not covering, the cervix and the woman has no bleeding, she may be allowed to rest at home — with instructions to call the doctor or hospital immediately if bleeding starts. Alternatively, bleeding that cannot be controlled will probably necessitate an immediate cesarean birth for the sake of the baby, even if the birth is preterm. Such a baby is probably better off in the hands of the skilled caregivers and the sophisticated equipment of a modern neonatal intensive care unit than inside the mother's uterus, where a bleeding placenta is no longer able to support it.
Outlook for the Future After Placenta Previa
Because in most cases placenta previa can be detected accurately before the fetus is in significant danger, it no longer poses the threat to babies and their mothers that it once did. Advances in technology such as the ultrasound test and other potentially life-saving measures, however, are useless without the prompt recognition of potential problems by the pregnant woman. Bleeding in the third trimester may not necessarily lead to serious problems if it is acted on, but it should never be ignored.