This occurs when the placenta has grown over the cervix, a condition that is usually identified before labor begins. Placenta previa is the most common cause of painless vaginal bleeding during pregnancy. In less than 2% of cases the previa does not bleed until labor begins.
About 5-6% of pregnancies have placental previa in the second trimester, but 90% of those will become normal before the baby is born. About 1/3 of pregnancies with a previa also have a baby in a "wrong" position such as transverse. This is because the low-lying placenta forces the baby into a different position.
Why is placental previa a labor challenge?
There are some risks to vaginal birth with a previa. One is the risk the the mother or baby will have excessive bleeding as the cervix opens and pulls away from the placenta (called hemorrhage). As the placenta pulls off the wall of the uterus, it becomes more difficult for the placenta to provide the needed oxygen to the baby.
Another concern with a low lying placenta is the risk the cord (which is attached to the placenta) will become compressed as the baby moves into position. The actual placement of the placenta has the most impact on the risk of cord compression.
There are variations in previa which influence the amount of risk a baby and mother face. A placenta may be near the cervix, but not covering it, or a previa may be completely covering the cervix. If the placenta is close, you will work with your caregiver to determine what safest course of action may be. When the cervix is covered, cesarean surgery is able to save the life of the baby (who would be cut off from oxygen) and the mother (who would risk hemorrhage as the placenta was torn away from the cervix).
Marginal previa comes to the cervix but does not block it.
Complete previa blocks the cervix.
Help the mother understand what is happening and how things may change closer to labor. 90% of second trimester previas become normally situated by the time labor begins.
Give the mother the freedom to express her fear and frustration without judging or correcting her. If she had hoped to give birth naturally and just discovered previa may interfere with her goals, she will have some normal grief and disappointment to deal with. Give her the space to work through that disappointment before expecting her to get on-board with making new plans.
Things to discuss with your caregiver:
Because so many previas identified early in pregnancy become normal by the end of pregnancy, it may be wise to hold off any decision making until closer to labor.
Bleeding can be frightening, but there is usually very little a caregiver can do about it. Though you have the option to try bedrest, it does not change the position of your placenta.
References:Enkin, Keirse, Nilson, Crowther, Duley, Hodnett and Hofmeyr. A guide to effective care in pregnancy and childbirth Third Edition. 2000. Oxford: Oxford University Press.