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What is the placenta?

Posted by Be Well

What is the placenta?
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The placenta is an organ, rooted to the lining of the womb, which links your baby's blood supply to your own, while also keeping the two separate. By linking to your blood supply, the placenta carries out functions that your unborn baby can't perform for itself.

Your unborn baby floats and moves in a bag of fluid called the amniotic sac, which is made of membranes. The placenta is linked to your baby by the umbilical cord.

What does the placenta do?

Oxygen and food pass from your blood supply into the placenta. From there, the umbilical cord carries the oxygen and food to your unborn baby. Waste products from the baby, such as carbon dioxide, are returned along the umbilical cord back to the placenta and then into your bloodstream.

Antibodies pass to the baby in the same way, to protect against infection. The placenta protects your baby against most bacteria; the few exceptions include those that cause syphilis and tuberculosis.

Viruses, however, can cross the placenta freely, for example, the rubella virus (German measles) which can cause miscarriage, stillbirth or birth defects such as deafness, brain damage, heart defects and cataracts.

Alcohol, nicotine and other drugs can also cross the placenta and can cause damage to your unborn baby.

The placenta produces hormones that help your baby to grow and develop. Towards the end of your pregnancy, the placenta passes antibodies from you to your baby, giving it immunity for the first three months after birth.

What happens after my baby is born?

After your baby is born, in the third stage of labor, more contractions will push the placenta out through the vagina. The placenta at this stage is also called the afterbirth.

You can choose to be given an oxytocic drug to stimulate contractions and help push the placenta out. Your midwife will inject the drug into your thigh just as the baby is born. It makes the womb contract so that the placenta comes away from the wall of your womb. This also helps to prevent the heavy bleeding that some women may experience.

You can also choose to let your body push the placenta out in its own time, which may involve some loss of blood.

After the birth, your midwife will inspect the placenta and membranes, to make sure they are complete and nothing has been left behind.


As your pregnancy progresses, the womb expands and this affects the placenta's position. The area where it's rooted usually stretches upwards, away from your cervix. However, if the placenta stays low in your womb, below your baby, it may cover part or all of your cervix and block the baby's way out. This is called low-lying placenta or placenta praevia. It happens rarely, affecting between three and six of every 1,000 pregnant women.

A low-lying placenta can be associated with painless, bright red bleeding from the vagina during the last three months of pregnancy - if this happens to you, contact your midwife or doctor immediately.

The placenta's position is checked during routine ultrasound scans. If your 18-23 week scan shows your placenta is low, this is usually no cause for concern as its position is likely to change. You will probably be offered another scan at around 36 weeks to check the placenta's position. If it remains in the lower part of your womb blocking the cervix, your baby cannot be delivered safely through the vagina, so you will need a caesarean section.

After your baby is born, part of the placenta or membranes can remain in the womb. This is known as retained placenta and happens in about 2% of births. Breastfeeding your baby causes the womb to contract, so this may help to push the placenta out. Your midwife may also ask you to change the position you are in, for example, by changing to a sitting or squatting position. If these methods don't work, it may be necessary for the placenta to be removed under general anesthetic.

Other rare complications can affect the placenta, such as placental abruption where the placenta comes away from the wall of the womb earlier than it should.

Always speak to your midwife or doctor if you are concerned about any aspect of your health when you are pregnant.

NOTICE: The information provided on this site is not a substitute for professional medical advice, diagnosis, or treatment. Never delay or disregard seeking professional medical advice from your physician or other qualified health provider because of something you have read on Wellsphere. If you have a medical emergency, call your doctor or 911 immediately.
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