When the placenta implants over or near the inner opening of the cervix, the condition is called placenta previa. The cervix is the opening of the uterus. As the cervix dilates during labor, the abnormal location of the placenta may cause heavy vaginal bleeding and keep the baby from traveling through the birth canal.
What is going on in the body?
Placenta previa occurs in 1 out of 200 births. In about 10% to 20% of these cases, the placenta covers the entire cervix. This is complete placenta previa. Otherwise, only part of the cervix is covered. This is called partial placenta previa.
Vaginal bleeding typically occurs:
as the lower part of the uterus starts to widen and thin out as the uterus grows in the later part of pregnancy
as the cervix opens during labor, which tears blood vessels in the placenta
with trauma to the placenta, as with the pressure of the penis during intercourse
The extent of the bleeding depends upon how much of the placenta covers the cervix and how many blood vessels are torn.
What are the causes and risks of the condition?
A woman is more likely to have this condition if:
she has had 6 or more births
she is carrying two or more fetuses
she previously had a cesarean section
she is older than 35 years
the placenta is large or abnormal