In the majority of pregnancies, the placenta forms in the top half of the womb. In some cases, however, it forms in the lower half, so that it lies partly or completely over the cervix, blocking the baby’s way out of the womb. This is known as a low lying placenta (or placenta praevia).
The position of your placenta can be shown by an ultrasound scan. If you have a scan in the first half of your pregnancy that shows that your placenta is low lying, this isn’t usually any cause for concern. As many as one in four placentas may be low lying in mid-pregnancy. The likelihood is that as your womb gets bigger, the placenta will move up (it doesn’t actually ‘move’ as such, but as the womb grows, the area to which the placenta is attached stretches upwards, away from the cervix). If your placenta is shown to be low lying in mid-pregnancy, you’re likely to be offered another scan later on (usually around 32-34 weeks) to check that it has moved. Only 0.5 per cent -1 per cent of placentas remain low lying at the end of pregnancy. However, when this happens, it can cause problems.
What problems does it cause?
The main problem that a low lying placenta can cause is bright red bleeding from the vagina during the later stages of pregnancy. The bleeding may stop and start at first, and is painless. It happens because the lower part of your womb is starting to change shape and thin out in readiness for your baby to be born, but because the placenta can’t stretch in the same way, it peels away at the edges, which causes bleeding.
At first, the bleeding is usually slight, but there’s a risk that it might become heavy at any time without warning. Heavy bleeding may be accompanied by signs of shock – paleness, sweating and an increased pulse.
If you haven’t had a scan that has shown the position of your placenta and you experience this kind of bleeding, it may be an indication that the placenta is low lying. Another sign that you might have a low lying placenta is that your baby is lying in an awkward position in your womb – for example from side to side, or at an angle.
Will it harm my baby?
Having a low lying placenta won’t in itself harm your baby. If it causes bleeding, the blood comes from the mother, not from the baby, although it’s occasionally possible that the baby might lose some blood too. Sometimes, however, if the bleeding is serious, it’s necessary to deliver the baby early.
What can be done about it?
Unless the low lying placenta causes bleeding, there is nothing that needs to be done about it. However, if you have any bleeding at any time, consult your doctor immediately. If he or she suspects that the bleeding is due to a low lying placenta, you will be advised to have an ultrasound scan. If the diagnosis is confirmed by the scan, the treatment will depend on exactly where the placenta is lying, how many weeks pregnant you are, and how much bleeding you’ve had.
If the placenta is lying across the cervix, there’s a very high risk that you’ll experience more bleeding, which could be extremely heavy and put both you and your baby’s lives at risk. Because of this you’ll almost certainly be advised to stay in hospital until your baby is born. There’s still a possibility that the placenta may move away from the cervix, but this becomes less likely as your pregnancy advances. You may be offered repeated scans to check whether the placenta is moving or not. If it doesn’t move, the only way that your baby can be born is by Caesarean. This will usually be carried out at around 38 weeks.
If the placenta is only partly over the cervix, you’ll probably still be advised to stay in hospital for the rest of your pregnancy because of the risk that you might bleed at any time. You’ll need to discuss with your doctor whether to have the baby normally or by Caesarean. Some doctors will always recommend a Caesarean because of the risk of bleeding with a normal birth.
What if I don’t want to have a scan?
You don’t have to have a scan if you don’t want to. However, if you’ve had any painless bright red bleeding, this does indicate that you may have a low lying placenta, and you’ll be advised to stay in hospital in case you bleed more.
A low lying placenta can also be diagnosed by an internal examination, but because there’s a risk that this can cause heavy bleeding, it’s only ever carried out in a hospital operating theatre, with preparations for surgery already done, so that the baby can be immediately delivered by Caesarean if necessary. For this reason, an internal examination will not be carried out unless the baby is mature enough to be born.