This page looks in more detail at placenta praevia and placenta accreta, two main causes of postpartum haemorrhage (PPH).
Placenta praevia (also called placenta previa)
There is a standard classification of placenta praevia, depending on how low the placenta is, ranging from Type I where the placenta is largely in the upper segment of the uterus and only slightly extending into the lower segment, to Type IV where the placenta entirely covers the opening of the cervix. This can be detected at an ultrasound scan late in pregnancy. If the placenta is near the opening of the uterus towards the end of pregnancy, the baby will have to be born by caesarean.
The cause of placenta praevia is unknown but the likelihood increases in women who have had many children, in multiple pregnancies and in women who have had a previous caesarean birth. Overall the incidence of placenta praevia is about 5 in 1000 pregnancies, but this doubles for a woman who has had a previous caesarean birth.
When the placenta is attached in the lower part of the uterus, there is a higher chance of bleeding when the placenta comes away after the birth of the baby. This is thought to be because the wall in the lower part of the uterus is not as effective at contracting as the wall of the upper segment, so the blood vessels are not sealed effectively.
If a woman is having a caesarean because of a placenta praevia, the medical team will be aware of the possibility of PPH and will be prepared for dealing with it.
This is where the placenta grows into the lining of the uterus which prevents it from detaching after the baby is born. It is most common in women who have had a previous caesarean birth and its incidence rises with the number of previous caesareans.
This occurs in about 0.04 - 0.1% of pregnancies, but the incidence has been rising in recent years because of the increase in caesarean births. Placenta accreta is often found in association with a low-lying placenta (placenta praevia).
Placenta accreta is not usually diagnosed until the third stage of labour when there is difficulty with delivering the placenta. At this stage, a postpartum haemorrhage is common as efforts are made to detach the placenta. Hysterectomy is very common in these circumstances - in some cases, where placenta accreta is suspected during pregnancy, plans may be made in advance to have a hysterectomy after the birth.
More recently it has been suggested that where the placenta fails to detach because of a suspected placenta accreta, that the placenta is left in place and antibiotics given. This is called conservative management. Because the placenta is not being pulled away from the uterus wall, this reduces the risk of bleeding. It is not know how common this is, and it requires careful follow-up from the medical team to ensure the safety of this treatment.