What is Placenta Previa?
Placenta previa is the term when the placenta implants low in the uterus (womb), either partially or completely covering the cervix, or it may not cover the cervix at all. It is an uncommon but major complication of pregnancy. A placenta previa can lead to bleeding during or immediately after delivery.
How does a placenta previa occur?
Implantation of the placenta in the lower parts of the uterus is not uncommon in the early stages of pregnancy while the uterus is relatively small. This changes during the course of pregnancy as the growing uterus redefines the position of the placenta in the distended uterus. By the second to third trimester of pregnancy, the placenta is located in the mid to top of the uterus. With placenta previa, however, this does not occur and the placenta is low lying in the uterus and may involve the cervix as well.
The placenta is the conduit for nutrients and oxygen from the mother’s blood stream to reach the fetus. It only dislodges after delivery of the baby as the contracting uterus shrinks to a smaller size and shears away the placenta. If the placenta is blocking the cervix, the baby cannot travel down the birth canal during delivery.
The uterine muscles are arranged in a manner that allows it to constrict the the blood vessels of the uterine wall. This ensures that uterine contractions after delivery constricts the mother’s blood vessels that were communicating with the placenta. This mechanism ensures that there is only minimal post partum hemorrhage and is further backed up by the action of clotting factors in the blood. With a placenta previa, however, this mechanism is disrupted as the cervix and lower segment of the uterus thins (atrophy) prior to delivery and the uterine contractions cannot constrict the blood vessels.
Types of placenta previa
Placenta can be one of three types :
- Marginal where the placenta is on the cervix, near the border of the os (cervical opening) but is not covering it.
- Partial where the placenta is only partly covering the cervical opening.
- Complete where the placenta is totally blocking the cervical opening.
What does a low placenta mean?
A low lying placenta is known as placenta previa and means that the placenta is located lower down the uterus near or over the cervix. Normally the placenta is positioned higher up the uterus by the second to third trimester. This ensures that the birth canal is unobstructed for delivery.
The placenta is a communication point for the circulation of the mother and fetus. Nutrients and oxygen are transported to the fetus via the placenta. It also delivers wastes product and carbon dioxide from the fetus to the mother’s blood stream so that it may be eliminated. The placenta needs to be firmly implant on the wall of the uterus (endometrium) so that it can maintain contact throughout pregnancy. After delivery, the uterine contractions detach the placenta so that it may be expelled through the birth canal. This detachment ruptures the mother’s endometrial blood vessels and various mechanisms take effect to limit bleeding and seal the vessels.
With a low placenta, these mechanisms may be hampered and can lead to bleeding during and after delivery. This compromises the blood supply to the fetus and also puts the mother’s health at risk due to excessive bleeding from the implantation site. Placenta previa may be marginal (near but not covering the cervical opening), partial (partly covering the opening of the cervix) or complete (totally blocking the opening of the cervix). At times, the term low-lying placenta may be used to describe a placenta that is in the lower half of the uterus yet does not encroach on the cervical opening. It is would therefore not qualify as a marginal placenta previa but can still cause bleeding after delivery.
What causes placenta previa?
A low placenta is more likely to occur in women with certain risk factors. This, however, does not mean that every woman with one or more risk factors will experience a placenta previa during pregnancy. These risk factors include :
- Multiparity – women who have had many children. This is is increased if there was a short time period between pregnancies or with multiple gestation (twins, triplets and so on).
- Endometrial scarring which may be due to previous surgery involving the uterus, previous pregnancies, cesareans sections, miscarriages, or induced abortions.
- Cigarette smoking, cocaine use.
- Abnormally shaped uterus.
What are the symptoms of placenta previa?
Vaginal bleeding and occasionally uterine contractions are the only two symptoms that will be seen with a placenta previa. It is may be diagnosed upon conducting a pre-natal ultrasound although the mother has not experienced any symptoms.
The characteristic feature of a placenta previa is painless, bright bleeding after the first trimester. It is more likely to be placenta previa if the bleeding occurs from the second half of the second trimester onwards. The vaginal bleeding may vary from light spotting to a heavy flow. It can be intermittent, stopping spontaneously and then recurring after a few days or weeks.
Some women with placenta previa will experience uterine contractions with the vaginal bleeding. The contractions may cause abdominal discomfort or even a cramping pain but this is not as intense as labor pains. Uterine contractions may only be seen in 1 out of 5 women with placenta previa.