Known as placenta previa, a low lying placenta is a pregnancy complication. The placenta is the organ that connects the maternal and fetal blood supplies. If it develops in the lower part the uterus, it can obstruct the cervix, through which a fetus must pass during a vaginal delivery. Normally, the placenta develops close to where the early embryo has implanted in the endometrium (the wall of the uterus), in the upper region of the uterus called fundus. However, if the embryo implants lower down, the placenta will develop lower down as well. The reasons for this are not clear, but one possibility is that there is a reduction in the number of blood vessels in the fundus, making it a less valuable place for implantation. Some researchers believe that this can result from previous damage to the fundus, during a spontaneous abortion (miscarriage) or a cesarean section (C-section). Another cause of placenta previa is multiple pregnancy. In most cases of twin pregnancies or pregnancies of higher numbers of fetuses, each fetus has its own placenta and there simply is not enough room for all embryos to implant in the fundus and for all placentas to attach at the fundus. Thus, one or more placentas end up attached lower.
Placenta previa can lead to severe hemorrhage (bleeding) and premature birth. Since the placenta blocks the cervix, vaginal delivery can cause a particularly severe hemorrhage, which can threaten the life of the mother. Severe hemorrhage, due to rupture of the placenta during delivery, can lead to perinatal death (death of the fetus just before or during delivery, or of the newborn just after birth). Even without placental rupture, placenta previa can cause premature labor and delivery, which increases the risk of medical issues for the newborn, including low birth weight and breathing problems.
Placenta previa develops in about 5 per 1,000 pregnancies. However, several factors can increase the chances of placenta previa. One such risk factor is having placenta previa in a previous pregnancy. Another risk factor is multiple pregnancy; if you are carrying twins, triplets, or a higher number of fetuses, placenta previa is more likely than if you have a singleton pregnancy. Other risk factors are maternal age over 35 years, smoking and cocaine use and, mainly, history of surgery on the uterus. The latter includes a previous cesarean section and removal of uterine fibroids.
Medical history and physical examination provide the obstetrician with important clues as to whether a woman may have placenta previa. If she had placenta previa in the past, this is a clue, as is a history of surgery on the uterus, including a C-section. Most important is what is happening to her in the current time. If she is experiencing heavy bleeding with little or no pain, this strongly suggests placenta previa. Some women with placenta previa may also experience premature contractions, but the obstetrician will use ultrasonography to confirm the diagnosis by actually visualizing the location of the placenta.
When a woman has placenta previa, most likely she should avoid a vaginal birth and should have the baby via C-section. This enables the baby to exit the mother without passing through the placenta that obstructs the cervix. To prevent pain during the C-section, she will be given epidural or spinal anesthesia. To accelerate maturation of the baby’s lungs, she may also be given corticosteroids, particularly a steroid called betamethasone, particularly if the need for cesarean birth is urgent and the pregnancy is not close enough to term for the lungs to be adequately developed already. Generally, this means that the delivery is to occur prior to about 35 weeks gestation, but in some cases corticosteroids may be given to mothers who are to deliver any time up to 37 weeks, just to be safe. Whatever the case, corticosteroids are safe for the fetus; in fact, the fetus is the target of these medications.