In a multiple pregnancy, there is more than one baby growing in the womb (uterus) at the same time. Being pregnant with twins is the most common example. Both mother and babies need extra monitoring during pregnancy as the risks are higher than in a singleton pregnancy. With specialist antenatal care, most pregnancies have a safe and successful outcome.
What is multiple pregnancy?
In a multiple pregnancy there is more than one baby growing inside the womb (uterus) at the same time. One baby growing in the womb on its own is called a singleton pregnancy. By far the most common type of multiple pregnancy is a twin pregnancy, when there are two babies. Having three babies in the same pregnancy is known as triplets, four is known as quadruplets. It is possible to have more than four but it is very rare.
There are several ways in which this can happen. Firstly, the babies can be identical or non-identical.
- Identical twins or triplets are called monozygotic. Mono means just one, and zygote means a fertilised egg. Therefore, monozygotic siblings come from one egg. In this case the egg has been fertilised by one sperm and then split into two or more embryos. These babies will all be the same sex. They will all have identical genes.
- Non-identical twins are called dizygotic twins (or fraternal twins). Triplets or higher multiples are polyzygotic. In this case each egg is fertilised by a separate sperm. The babies are not necessarily the same gender and they have different genes.
When an egg (or more than one egg) is fertilised, it becomes a zygote. It immediately divides into two cells and each of those cells then divides. This continues until there is a ball of cells which implants into the lining of the womb and becomes an embryo. Each embryo will develop into a fetus and then eventually become a baby.
As the early ball of cells implants itself into the womb, the lining of the womb starts growing another type of tissue called the placenta. The growing embryo is attached to the placenta through a tube called the umbilical cord. The placenta supplies all the nutrients and blood to the growing embryo/fetus. The fetus grows within a bag of fluid called the amniotic sac. The inner lining of the amniotic sac is called the amnion. The outer lining is called the chorion, and connects to the placenta.
Dizygotic twins have their own placenta, amnion and chorion. So too do all other non-identical triplets or higher multiples. Each fetus develops separately in its own sac with its own blood supply. Each placenta may be separate or fused together.
In identical (monozygotic) pregnancies, however, this situation is more complicated. It depends on exactly when the zygote divided into two or more zygotes. There may be one placenta or more than one. If there is one shared placenta, this is called a monochorionic pregnancy. The word chorionicity describes how many chorions there are. There may also be one amnion or more than one. The word to describe this is amniocity. If there is one amnion, it is called a monoamniotic pregnancy. As a result there can be various different types of monozygotic pregnancies - for example:
- Twins can be dichorionic diamniotic. The twins have separate amnions and chorions. This is the type of twin pregnancy with least risk.
- Twins can be monochorionic diamniotic. The twins share the same chorion and placenta. They grow in separate amniotic sacs.
- Twins can be monochorionic monoamniotic. The twins share the same amnion, chorion and placenta.
- Siamese twins (conjoined twins). The fetuses themselves are joined together physically. It is very rare.
- Triplets can be:
- Trichorionic. Each baby has its own placenta and chorion.
- Dichorionic. Two of the babies share a placenta and chorion and the other is separate.
- Monochorionic. All three babies share the same placenta and chorion.
- In separate amniotic sacs, or one or more baby can share an amniotic sac.
These different types of multiple pregnancy have different risks and problems attached to them. Antenatal care is therefore different depending on which type of multiple pregnancy you have. Generally, monochorionic pregnancies are considered higher-risk. The more babies there are, the higher the risk.
Why does multiple pregnancy occur?
Multiple pregnancy can and does occur naturally. In some cases the woman produces more than one egg (ovulates). In others there is just one egg fertilised but it splits into two zygotes. However, multiple pregnancy is more likely after assisted reproductive techniques such as in vitro fertilisation (IVF). This is because, traditionally, more than one embryo was transferred to the womb (uterus). This was to increase the chances of one surviving to become a baby.
How common is multiple pregnancy?
Multiple pregnancy is more common in some parts of the world than in others. It is least common in Japan and most common in some countries in Africa. In the UK it occurs in around 16 out of every 1,000 births. Having twins is by far the most common type of multiple pregnancy. Of those 1,000 births, around 986 would be twins and 13 would be triplets. Having quadruplets or more is very rare.
Multiple pregnancy has become more common in the UK over the last 30 years. This is because IVF has been used increasingly. Before IVF became common, about 10 in 1,000 births were multiple. Now this has increased to about 16 in 1,000. The Human Fertilisation and Embryology Authority (HFEA) has set targets to lower the numbers of multiple pregnancies which occur due to IVF. This is because being pregnant with more than one baby is more risky than being pregnant with one baby.
Who can have a multiple pregnancy?
Any woman who can become pregnant could have a multiple pregnancy. It is more likely if:
- You have had an assisted reproductive technique such as IVF.
- You are over the age of 45 when you become pregnant.
- You have a family history of twins on your mother's side.
- You are of West African origin.
- You have had a multiple pregnancy before.
Is antenatal care different if I am pregnant with twins or triplets?
Yes. If you are pregnant with more than one baby, you will have more intensive antenatal care. This means you will have more check-ups and more ultrasound scans. The exact regime will depend on how many babies you have inside your womb (uterus). It will also depend on whether the babies share a placenta or not.
If you are pregnant with more than one baby, this is usually picked up at your first ultrasound scan. For most women, this would be at 11-13 weeks of pregnancy (11-13 weeks after the first day of your last period.) Women who have had IVF will have had an earlier scan and will know sooner. Your antenatal care will then usually be in the hands of a specialised team. This team is made up of the person who does your scan (the ultrasonographer), the midwife who looks after you and a doctor who specialises in pregnancy and childbirth (an obstetrician). These specialists will all be experienced in looking after women with multiple pregnancy. If needed, you may also be referred for advice from other relevant specialists. This might include a physiotherapist, a dietician, a baby feeding specialist or a mental health specialist. In certain pregnancies, considered higher-risk, you will be cared for by a specialist "fetal medicine centre." This is more likely if you are having three or more babies, or if any of your babies share a placenta.
At 11-13 weeks of pregnancy, you will have the same screening ultrasound scan all pregnant women have. This is to check the babies' age, whether or not they share a placenta and how many there are. It also is part of the check for Down's syndrome, if you choose to have this check. The scan is exactly the same as for women with one baby; however, it does take longer as there are two babies to check. The team in your antenatal clinic will explain to you what this involves.
Deciding whether to have the test for Down's syndrome is more complicated for multiple pregnancies. You will need extra information to help you. The test for Down's syndrome involves a combination of the ultrasound scan and a blood test. If you have identical (monozygotic) twins, the risk is the same for each twin. However if your babies are not monozygotic, the risk of Down's syndrome will be different for each. It is not possible to be as accurate in determining this risk in multiple pregnancies as it is when there is just one baby. It may be that one baby has a high risk of Down's syndrome but the other does not. Your antenatal team will give you all the information you need to be able to decide whether to have the test or not.
After this, the number and frequency of your scans will depend on what type of pregnancy you have. That is, how many babies and how they are connected to each other (chorionicity and amniocity as discussed earlier). You will have more scans than a woman who has a straightforward pregnancy with one baby. This is so that your antenatal team can check your babies are growing normally. It is also to check for a condition called feto-fetal transfusion syndrome. This is explained further below, in the "What is twin to twin transfusion syndrome?" section.
You will have blood tests at least twice in your pregnancy to check you are not becoming anaemic. This occurs when the level of red blood cells in your blood becomes too low. It is common in pregnancy and even more common in multiple pregnancy.
Some women may be advised by their specialist to take a low dose of aspirin. This is particularly for women who are at risk of high blood pressure in pregnancy. Your specialist will advise if this applies to you.
The general advice about diet and lifestyle in pregnancy applies to women who are pregnant with more than one baby.
Which kind of birth will I have for twins or triplets?
If you are pregnant with twins or triplets, it is strongly advised you deliver in a hospital. This is because of the extra risks involved. If you are in a hospital setting, if there is a problem, it can be more rapidly diagnosed and dealt with. This way you are more likely to have a safe delivery.
Whether you have a vaginal delivery or delivery by an operation (a caesarean section) will depend on a number of factors. This includes:
- Your preference.
- How many babies you are having. Triplets are almost always delivered by caesarean section.
- Which way round your babies are. If the first baby is "head down" near the time of delivery, you may usually choose to have a vaginal delivery. (Unless they share a placenta, in which case you would normally be advised to have a caesarean section.)
- Whether the babies share a placenta or not.
- Whether you have had a caesarean section in the past or not.
- Whether there are any other complications of the pregnancy.
- Whether there are any problems that come up during labour. If the heart rate of either baby slows down, it may become vital to get the babies out quickly.
You and your specialised team will discuss the options and decide on the safest method of delivering your babies. This may need to change if the situation changes - for example, if one of the babies changes position, or the heart rate monitor suggests a problem.
When will my twins or triplets be born?
Usually twins and triplets are born earlier than most single babies. This is because your womb (uterus) is being stretched more and because the space for the babies to grow is limited. There is only so far it can stretch before labour is triggered. Also, the babies may stop growing if they continue inside until your normal due date.
Your antenatal team will usually offer you the option of being induced early (or an early planned caesarean section). This will depend again on how many babies you have and how they are connected (whether they are monochorionic or dichorionic). Early delivery may be advised at 35, 36 or 37 weeks of pregnancy. If you choose not to have a planned early delivery, your babies will be monitored very carefully each week to check they are growing normally. If you have a very early delivery, you may be offered a course of steroids. This helps the babies' lungs to develop, so that they can breathe better following delivery.
What are the risks associated with having twins or triplets?
Many women who are pregnant with more than one baby have normal pregnancies and no problems. However, it is considered higher-risk compared to being pregnant with one baby. This is why you will be monitored so much more intensively. If you are pregnant with more than one baby, there are more risks both to you and to your babies, than in a singleton pregnancy. The risks are higher for babies who share a placenta (monochorionic pregnancies).
Risks for the mother
If you are having more than one baby, your pregnancy symptoms tend to be worse. That is, you are more likely to have morning sickness and it is more likely to be a problem. You are more likely to develop a syndrome of severe morning sickness, called hyperemesis gravidarum. You are more likely to have heartburn. You are likely to become breathless earlier, as your expanding womb (uterus) presses up on your lungs. You are more likely to have problems associated with more pressure on your blood vessels - for example, piles or varicose veins.
You are more likely to develop a condition of high blood pressure in pregnancy, called pre-eclampsia. This is three times more common in mothers of twins than in mothers of singleton babies. For mothers of triplets, it is nine times more common. This is one of the reasons you will have more frequent antenatal checks. Your blood pressure is monitored carefully, along with testing of your urine for protein. If you do develop any problems, these can be picked up and treated early. As discussed above, anaemia is also more common in multiple pregnancy. This will also be monitored. You will be advised to take iron tablets if you are found to be anaemic.
Many other conditions are more common in multiple pregnancy than in singleton pregnancies. This includes:
- Having a delivery by operation (caesarean section).
- A condition where there is too much fluid around the baby, called polyhydramnios.
- Excessive bleeding (haemorrhage) before, during or after delivery.
- Postnatal problems. These could be physical health problems, mental health problems, or breast-feeding-related problems. There can also be financial problems with more than one new baby. There may also be practical problems caring for more than one baby.
The risk of dying as a result of pregnancy is very small nowadays in the UK. However, it is more than twice as likely for mothers carrying more than one baby as it is for those with one baby. It is still highly unlikely. You and your antenatal team work together to keep you safe and healthy by having regular check-ups.
Risks for the babies
In multiple pregnancy there is a higher chance of the following:
- Stillbirth. In pregnancies where there is one baby, the risk of stillbirth is around 5 per 1,000 births. For twins, this rises to around 12 per 1,000 births. For triplets, this is higher still at 31 per 1,000. The risk is higher where the babies share a placenta (monochorionic). Overall it is still a low risk.
- Premature birth. Babies are more likely to be born early, either by natural early onset of labour or because it is advisable to choose to deliver them early. Babies born too soon have a higher likelihood of having long-term developmental problems. As newborns, they are also at higher risk of not surviving.
- Tangled umbilical cords. This may happen to babies who share a placenta and amniotic sac.
- Poor growth.
- Congenital abnormalities such as Down's syndrome. This is currently the subject of debate. It is thought that there is a higher risk of congenital abnormalities in monozygotic twins and triplets than in singleton babies. However, some recent studies question this.
There is also the risk of feto-fetal transfusion syndrome. This is explained further in the section below.
Your antenatal team will be very aware of all these risks. It may sound a bit scary but this is why you have extra special care. You have more check-ups and more scans so they can keep an eye on you and your babies. In this way, if anything is going wrong, it can often be corrected or treated. This is why it is very important to attend all your appointments. Together, you and your antenatal team can increase the chance of a safe delivery of healthy babies.
What is twin to twin transfusion syndrome?
In feto-fetal transfusion syndrome, one of the babies receives more of the blood supply. The other baby or babies may not get enough and may fail to grow. In the case of twins, this is called twin to twin transfusion syndrome (TTTS). The twin who is not getting enough blood does not grow as well as the other twin and is at risk of not surviving. It usually happens in monochorionic pregnancies, where there is a shared placenta.
If you are pregnant with more than one baby, you will have regular scans to watch out for TTTS. If TTTS is picked up, there are a number of possible treatment options. The usual treatment is laser surgery to the blood vessels through which the babies share blood.
Further help & information
Dr Mary Harding
Dr Mary Harding
Dr Hayley Willacy