In pregnancy, the placenta imbeds into the uterine wall, usually in the top part of the uterus. The role of the placenta is to deliver oxygen and nutrients to the baby, and to remove the baby's waste products, which are then filtered by the mother’s liver. Although the placenta usually implants in the upper part of the uterus, sometimes it implants in the lower part. This can be detected in an ultrasound at 19 weeks, and at this stage is known as a low-lying placenta. If your midwife or obstetrician finds that your placenta is low-lying, s/he will usually organise for you to have another ultrasound at around 32-34 weeks. If, at this stage, your placenta is still low, then it would be called placenta praevia. There can be problems during labour, for example if the placenta completely covers the cervix, because in this case, as the labour progresses and the cervix dilates, the placenta separates and the baby loses oxygen and nutrients. For this reason, if the placenta is covering the cervix later in pregnancy, a caesarean is the only safe way for the baby to be born.
The good news is that the majority of placentas are taken up by the uterus as the uterus grows, so that by the end of pregnancy, most placentas are out of the way of the cervix and so normal labour and birth would be the safest option.
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