The placenta, which is eventually discharged from the mother's body as the afterbirth, is vital to the baby's continuing wellbeing in the uterus. Problems concerning the placenta can therefore have important consequences for the baby, during both the pregnancy and the birth.
This condition occurs when the placenta is situated in the lower part of the uterus. The main danger is major bleeding which normally occurs after about 30 weeks. However, although the 20-week scan often shows the placenta to be'low lying', in most cases the placenta moves upwards, out of the way of the cervix. As a result, by late pregnancy, fewer than 1 per cent of pregnancies show placenta praevia.
Risk factors for placenta praevia include:
- Previous placenta praevia
- A previous caesarean section " Pregnancies where there is a larger than usual placenta (for example, twin pregnancies).
Classic symptoms include recurrent episodes of painless, heavy vaginal bleeding. Why this should happen before labour is a mystery, but during labour it is the result of the placenta lying over the dilating cervix and coming away from the uterine wall.
Ultrasound is usually a straightforward means of diagnosis. In some instances, particularly when the placenta is at the back of the uterus, a transvaginal scan will show how close the leading edge of the placenta is to the cervix. Another indication of placenta praevia, apart from bleeding, is the failure of the baby to engage into the pelvis - instead he adopts a variety of positions (referred to as'unstable lie') because the placenta is preventing him from adopting the normal 'head-down' position of late pregnancy.
A blood transfusion may be necessary if bleeding is excessive. The mother-to-be may be advised to rest and also to avoid sexual intercourse. A placenta that is very close to the cervix will prevent a normal delivery, so a caesarean section will be advised.
Abruption occurs when the placenta becomes detached from the wall of the uterus. This occurs in about 1 per cent of pregnancies and results in maternal bleeding, which may or may not be apparent, depending on how close to the cervix the placenta is situated.
Risk factors for placental abruption include:
- Previous placental abruption
- Abdominal injury a Smoking oUse of cocaine
- Pregnancy later in life
- Several previous pregnancies
The classic symptoms of placental abruption include severe abdominal pain and bleeding. With a severe abruption the uterus will be tender and rigid. The amount of blood lost through the vagina may not accurately reflect the true amount of bleeding, because much of the blood can remain concealed within the uterus. This is a risk not only to the baby, because of the placenta becoming detached, but also to the mother, who may develop major problems with blood clotting.
Treatment depends on the severity of the abruption. A mild abruption may only require a period of observation in hospital to check that both the baby and mother are doing fine. More severe cases may require resuscitation of the mother and early delivery.
The third stage of labour covers the birth of the baby to the delivery of the placenta. In 1-2 per cent of pregnancies the placenta fails to come out, despite carefully pulling on the umbilical cord. In these circumstances, the mother is at risk of bleeding because the uterus is unable to contract properly.
Sometimes a short wait (up to 30 minutes or so) resolves the problem. If not, it may be necessary to remove the placenta manually. This is carried out under anaesthetic (usually an epidural or a spinal) and the
obstetrician gently inserts his fingers into the vagina and can then separate the placenta from the wall of the uterus. Once the placenta has been removed, the mother will be given oxytocin intravenously to help the uterus contract and thus reduce further blood loss. Antibiotics will usually be recommended because of the increased risk of infection.
Any major haemorrhage can disturb the blood-clotting system.These disturbances are more common after a placental abruption because the amount of bleeding may be very much greater than that revealed. Other conditions, such as preeclampsia (in which abruption can occur), can also directly affect the mother's blood-clotting system.