The Role Of The Placenta In Pregnancy

pregnant_woman_having_an_appointment_with_a_doctorThe placenta plays a crucial part in pregnancy, and greatly influences the health of both mother and baby, writes Associate Professor Andrew Bisits.

What Is A Placenta And What Does It Do?

The placenta is an amazing organ that plays a crucial role in pregnancy in keeping the baby alive and well. When it is delivered after the birth of the baby, it looks somewhat like a piece of liver, and usually weighs about one sixth of the baby's weight.

The main function of the placenta is to supply the baby with adequate nutrition. Blood from the mother goes through the placenta then into the umbilical cord attached, before reaching the baby. This blood contains oxygen, glucose and an array of other nutritional substances. Before the blood goes to the baby, the placenta works like a kidney to filter it of harmful substances, while also letting those that are good for the baby pass through.

As well, the placenta acts like a lung for the baby, allowing the transfer of oxygen just like the lungs do in a newborn baby or adult. Once the baby has used the blood with the oxygen, the blood goes back to the placenta to get rid of carbon dioxide and pick up more oxygen. The placenta also ensures these waste products from the baby are released into the mother's circulation, which in turn are disposed of through her urine. It also helps to protect the baby from infections because it keeps the mother's blood and baby's blood separate by acting as an effective filter.

The placenta produces a long list of hormones. The hormone produced in largest amount is human placental lactogen, which causes the mother to have more glucose in her circulation to pass on to her baby. The placenta also produces vast amounts of the female hormones oestrogen and progesterone. These hormones have a role in stopping any contractions in the uterus before the baby's birth and also in preparing the uterus and maternal tissues for labour. They are also responsible for some of the changes that occur in a woman's body during pregnancy. Some of the hormones, namely oestrogen and corticotrophin-releasing hormone, are thought to influence the timing of the onset of labour.

During pregnancy, the placenta moves as the womb stretches and grows. It is very common for the placenta to be low in the womb in early pregnancy, but to move to the top of the womb as the pregnancy continues, so the cervix is open for delivery by the third trimester. In most pregnancies, the placenta attaches at the top or side of the uterus.

The period from just after the baby is born until just after the placenta is expelled (which is usually within 15 to 30 minutes of birth) is called the third stage of labour. This can be managed actively, for example with an injection of oxytocin followed by cord traction to assist in delivering the placenta, or it can be expelled without medical assistance.

What Affects The Health Of A Placenta?

Although it is a very robust foetal organ, various factors can affect the health of the placenta during pregnancy and increase the risk of certain problems, some of which can be modified and some that can't, such as:
•    Abdominal trauma: trauma to the abdomen, such as from a fall or other type of blow to the abdomen.
•    Blood-clotting disorders: any condition that impairs the blood's ability to clot or increases the likelihood of clotting.
•    High blood pressure
•    Maternal age: problems are more common in older women, especially those over the age of 40.
•    Multiple pregnancy.
•    Premature rupture of the membranes: when the fluid-filled membrane called the amniotic sac that surrounds and cushions the baby leaks or breaks before labour begins.
•    Previous placental problems: women who have had a placental problem during a previous pregnancy.
•    Previous uterine surgery: previous surgery involving the uterus, such as a caesarean section.
•    Substance abuse

The best test for the functioning of the placenta is an ultrasound along with Doppler measurement of blood flow through the umbilical-cord blood vessels. While there is no specific check-up to look at the health of the placenta, women with gestational diabetes, high blood pressure or those whose babies have small measurements in routine antenatal care, will have a scan to check the blood flow between the foetus and the placenta in the final trimester.

Problems In Pregnancy Related To The Placenta

• Pre-eclampsia/blood pressure: this common problem, which affects up to 10 per cent of women in their first pregnancy, originates in the placenta, and occurs when the placenta has not developed normally.

If a woman experiences significant blood pressure during pregnancy, labour or after birth, then she will need medication to treat and lower her blood pressure. In addition, the woman has to be assessed to see if her kidney, liver, blood clotting and central nervous system have been affected. The definitive treatment for a woman with pre-eclampsia during pregnancy is delivery of the baby. This is generally best if a woman has a vaginal birth, but there may be instances where a caesarean section is required.

• Placental abruption: this is a condition where the placenta has not developed normally and eventually separates from the wall of the uterus during the pregnancy.  

In severe cases, the baby has to be delivered usually between one and two hours. This will often be by caesarean section, however there are a good number of women who labour very quickly and the baby is born vaginally.

• Placenta accreta: this occurs when the placenta attaches itself too deeply to the uterine wall. The specific cause of placenta accreta is unknown, but can be related to placenta praevia and previous caesarean deliveries. A caesarean delivery increases the possibility of a future placenta accreta, and the more caesareans, the greater the increase in possibility.

Women with placenta accreta will most likely need to have their baby delivered by caesarean section, and special attempts will need to be made to control any extra bleeding immediately after, which usually occurs. If the bleeding cannot be controlled, a hysterectomy may need to be considered.

• Placental insufficiency: this occurs when the placenta is not working well enough. It can occur in women with high blood pressure, women who smoke and in association with some chronic diseases such as diabetes or kidney diseases. Placental insufficiency often results in the birth of a smaller baby.

Once detected, the baby is monitored more closely throughout the pregnancy with scans. Once the baby has reached a gestational age of about 36 to 37 weeks, birth is recommended.

• Placenta praevia: a problem of pregnancy in which the placenta grows in the lowest part of the uterus and covers all or part of the opening to the cervix, which is the opening to the birth canal. The main symptom of placenta praevia is sudden bleeding from the vagina, with some women also experiencing cramping. The bleeding often starts near the end of the second trimester or beginning of the third trimester.

There are three different forms of placenta praevia: marginal (where the placenta is next to the cervix, but does not cover the opening); partial (where the placenta covers part of the cervical opening); and complete (where the placenta covers all of the cervical opening).

Nearly all women with placenta praevia need a caesarean section. If the placenta covers all or part of the cervix, a vaginal delivery can cause severe bleeding, which can be dangerous to both the mother and the baby.

Did You Know?
•    An interesting feature about the placenta is that it has no nerve cells and therefore is not directly under the control of the brain or spinal cord.
•    Every minute during pregnancy, one pint, or 568.26ml, of blood is pumping into the uterus, exchanging nutrients with the placenta.
•    The food a mother eats during pregnancy does not go directly to the baby. It is broken down into tiny particles of proteins and nutrients that cross the placenta.
•     Babies do not breathe amniotic fluid. Oxygen also diffuses across the placenta into the foetal blood, travelling through the umbilical cord into the baby's circulation.


About the author: For the past 28 years, Associate Professor Andrew Bisits has been involved in various aspects of public obstetric and gynaecological practice. He is currently the medical co-director of the Maternity Services Division at the Royal Hospital for Women in Sydney, where he has an active obstetric practice and is involved in management, research and teaching of obstetrics.

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