Placenta: what it is, how it works, what's normal and what's not

Your developing baby is connected to the placenta by their umbilical cord – and together, the placenta and umbilical cord act as a lifeline for your baby, supplying them with nutrients and oxygen.

What does the placenta do?

The main roles of the placenta are:

To deliver oxygen, nutrients and antibodies from your blood to your developing baby
To transfer carbon dioxide and waste products, such as urea and creatinine, back from your baby to your blood – so that your body can get rid of them.
To be a selective barrier between your cells and your baby's cells, so that most microbes, including bacteria and viruses, can't cross over from your blood to your baby. A small number of viruses, including rubella and cytomegalovirus (CMV) can evade this barrier.
To transfer antibodies from your blood plasma to your baby to give them some immunity from infection in their first few months of life.
To produce hormones, such as hCG (human chorionic gonadatrophin, the hormone that pregnancy tests are designed to detect) and extra amounts of oestrogen and progesterone. This turbo-boosting of hormone production stimulates your baby's growth and development and triggers your blood volume to increase and your womb and breasts to enlarge.
To secrete substances that protect your baby from being treated by your immune system as a 'foreign body' that should be rejected.

When does the placenta start to form and grow?

The placenta starts to develop about 7 to 10 days after conception – at the same time as your fertilised egg is implanting itself in your womb.

It starts off as just a small collection of cells. It grows throughout your pregnancy: by the time your baby is born, it will be about 25cm long, 3cm thick at its centre and weigh about 450g.

Can substances cross the placenta that are bad for my baby?

Yes. As well as oxygen, nutrients and antibodies (going to your developing baby) and waste products (coming back from your developing baby), other substances can cross the placenta – and potentially have an impact on your baby's health.

These substances that can cross the placenta include drugs, alcohol, nicotine and some medications. This is why it's recommended that you don't smoke or drink alcohol are when you're pregnant, and why you should always check with a pharmacist that any over-the-counter medications you're thinking of buying are suitable for use in pregnancy.

Some infections, such as rubella, CMV (as mentioned above), mumps and toxoplasmosis can cross the placenta, too. If you think you've been exposed to any of these, do check with your doctor.

One thing that doesn't cross the placenta is blood – so your blood and the blood of your developing baby do not actually mix during your pregnancy. Instead, the placenta acts as an interface between the two.

Where exactly in the womb does the placenta form?

Pretty much anywhere! It all depends where on the wall of your womb your fertilised egg implants itself. Doctors, sonographers and midwives tend to talk about 4 main positions for the placenta:

Posterior placenta. This is where the placenta grows on the back wall (the spine side) of your womb.
Anterior placenta. This is where the placenta grows on the front wall (the tummy side) of your womb.
Fundal placenta. This is where the placenta grows on the top wall of your womb.
Lateral placenta. This is where the placenta grows on the left or right wall of your womb.

Whatever aspect of your womb the placenta attaches (apart from the top), it can sometimes form low down. If you have a low-lying placenta, it is often picked up at your 20-week scan.

Low-lying placenta can present a problem if some of the tissue grows over your cervix (something that's called placenta praevia – see Can there ever be problems with the placenta?, below). But, often, as your womb grows and stretches over the subsequent weeks of pregnancy, the placenta can move higher and away from your cervix, correcting the problem by itself.

What if I'm expecting twins? Will I grow two placentas?

It depends what kind of twins you're expecting.

With fraternal twins, there are two separate eggs, fertilised by two separate sperm, so, yes, there will also be two placentas.

In the case of identical twins, there is one sperm and one egg, which then separates into two. Identical twins can have one placenta each or they can share one placenta between them.

Can there ever be problems with the placenta?

In most pregnancies, the placenta develops and functions without any issues. However, problems with the placenta can sometimes occur. These include:

Placenta praevia – where the placenta forms low in your uterus, doesn't rise up as your pregnancy progresses and is partially or totally covering your cervix. If it is totally covering your cervix, your baby will need to be delivered by Caesarean section. Placenta praevia can also cause bright red bleeding during pregnancy (usually after 20 weeks) and labour.
Placental abruption – where the placenta either partially or totally comes away from the wall of the uterus before your baby is born. This can lead to bleeding during pregnancy and labour.
Placental insufficiency – where the placenta is not working as it should, meaning that your developing baby isn't getting enough of the oxygen and nutrients that are needed.
Placenta accreta – where the placenta grows too deeply on the wall of your womb too and cannot detach as expected after your baby is born, leading to bleeding
Retained placenta – where part of your placenta stays inside your womb after your baby is born, either because of placenta accreta (above) or because your cervix closes or your womb stops contracting.

The most common signs of a problem with the placenta are bleeding and abdominal pain. If you experience either during your pregnancy, please contact your medical team urgently.

What happens to the placenta when my baby's born?

Once your baby is delivered, your placenta is next up! In fact, delivery of your placenta – often called 'afterbirth' at this point – is called the third stage of labour .

If you've had a vaginal delivery, then shortly after you have pushed your baby out, you will have further contractions. Your midwife may push on your belly to help the placenta come out or they may ask you to push one more time. Medications can be given to help bring this along.

If you've had a Caesarean section, the surgeon will remove the placenta after delivering your baby.

In either situation, a midwife or doctor will examine the placenta to check that it looks intact and complete. This is because if any placenta is left inside your womb (retained placenta – see Can there ever be problems with the placenta?, above), it can cause issues such as bleeding or infection.

Why do some people eat the placenta?

Some people believe that eating part of the placenta, once it's been expelled from your body, can be good for your health. They eat it raw or cooked or even, as in Kim Kardashian's case, dried and made into pills.

There is no scientific evidence of any nutritional or mental-health benefits from eating placenta.

Why do some people bury the placenta?

In some cultures – including Maori culture and Navajo culture – burial of the placenta is an important post-birth ritual. It's often linked to ensuring the future health of both the mother and baby. For native Hawaiians, it's traditional to bury the placenta and then plant a tree on top, so both baby and tree can grow together.

Pic: Getty Images

Dr Philippa KayeGP

Dr Philippa Kaye works as a GP in both NHS and private practice. She attended Downing College, Cambridge, then took medical studies at Guy’s, King’s and St Thomas’s medical schools in London, training in paediatrics, gynaecology, care of the elderly, acute medicine, psychiatry and general practice.

What is the placenta?