Placenta praevia is a situation that occurs in about 0.5% of pregnancies where the placenta, instead of attaching itself to the side of the womb, lies over part or the whole of the cervix at the bottom of the uterus. This is a problem since the cervix (neck of the womb) is essentially the exit through which the baby is due to be born. In addition, as the womb enlarges and stretches to accommodate the growing foetus, the many blood vessels of the placenta become stretched and may tear, resulting in bleeding.
The amount by which the placenta is covering the cervix is graded into four grades:
- Grade 1 is where the lowest edge of the placenta is attached to what is called the lower segment of the uterus which is the area of the uterus surrounding the cervix.
- Grade 2 is where the lowest edge of the placenta reaches, but does not cover, the internal os (the cervical exit from the womb).
- Grade 3 is where the placenta partially covers the internal os.
- Grade 4 is where the placenta fully covers the internal os therefore completely blocking the exit from the womb.
The significance of these grades is that grades 1 and 2 are considered minor degrees of placenta praevia because they often correct themselves as the pregnancy progresses or may not prevent a normal vaginal delivery. However, grades 3 and 4 correspond to major degrees of placenta praevia because they are unlikely to correct themselves and almost always prevent a normal vaginal delivery from taking place.
If placenta praevia remains undiagnosed until labour starts (unlikely in these days due to the use of ultrasound scans) there may be severe bleeding risking the life of the mother and baby.
The cause of placenta praevia is not known but is more common in women who smoke, are over the age of 35 and in women who have had surgery on their womb such as a previous Caesarean section. It is also more likely to occur in multiple pregnancies such as twins or triplets. Women who have had placenta praevia once have a slightly increased chance of having it occur in a subsequent pregnancy.The most common symptom of placenta praevia is painless vaginal bleeding in the second half of pregnancy. A subsequent ultrasound scan will show the placenta to be attached low down in the womb either fully or partially covering the cervix.
Quite frequently placenta praevia will have been discovered on a routine scan done in early pregnancy before it has caused any symptoms at all. On these occasions, frequent follow-up scans are performed and quite often the placenta will be seen to gradually move away from the cervix as the womb enlarges, particularly in cases of grade 1 or 2 placenta praevia, in which case the problem may correct itself.
If placenta praevia continues to the third trimester of pregnancy or if any bleeding is likely to be putting the health of the mother or baby at risk, the baby is usually delivered by either planned or emergency Caesarean section depending on the circumstances.