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.
What is multiple pregnancy?
A multiple pregnancy is a pregnancy in which you are having more than one baby at the same time. One baby growing in the womb (uterus) 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 and having four is known as quadruplets. It is very rare to have more than four.
What is the difference between identical and non-identical twins?
Non-identical twins are called dizygotic twins (or fraternal twins). Triplets or higher numbers of babies are called polyzygotic twins, and this happens either:
- Because you produced more than one egg when you ovulated; or
- You had assisted conception and more than one fertilised egg was put into your womb (uterus).
In non-identical twins each egg is fertilised by a separate sperm. The babies have different genes from one another and are not necessarily the same gender. They are a genetically different pair of siblings growing in the womb at the same time.
Non-identical twins are the more usual type of twins. Having this sort of twins runs in families (passed down the female side of the family), and is more common if you are a particularly young mother or an older mother.
Identical twins or triplets are called monozygotic. 'Mono' means just one, and 'zygote' means a fertilised egg, so monozygotic means 'one fertilised egg'. Monozygotic siblings come from one single egg. Very early in its development this egg splits into two (or more) separate, baby-generating zygotes. Monozygotic babies are genetically identical to one another, so they will all be the same sex, will all have identical genes and will usually look very similar as they grow up. In Shakespeare's day people believed that identical twins could be of different gender (the play 'Twelfth Night' is based on this idea) but we now understand why this can't happen.
Identical twins do not run in families.
What happens when an egg is fertilised?
When an egg is fertilised, it becomes a zygote. It immediately starts dividing, quickly increasing the number of cells. If more than one egg is available to be fertilised then two (or more) separate zygotes may start dividing side by side.
If it splits completely into two separate groups of cells very early in this division then identical twin zygotes are formed. Cell division continues until each zygote has become a ball of cells which implants (attaches and embeds) into the lining of the womb and becomes an embryo. Each embryo will eventually become a baby.
As the balls of cells implant into the wall of the womb (uterus), they meet up with some specialised cells from the mother's womb and, between them, start to form the afterbirth (placenta). The placenta is crucial to the pregnancy - it is the place where your body 'meets' your baby directly to pass across the nutrients your baby needs to grow. Although your blood and your baby's blood do not actually mix with one another (ie you don't share the same circulation), in the placenta your blood and your baby's blood come close together.
The ball of cells now starts to form distinct areas, including a central group of cells (which will form your baby) and the outer cells which will form the amniotic sac (membranes). The membranes have two layers - an inner lining (called the amnion) and an outer lining (called the chorion).
What types of twin pregnancy are there?
In identical (monozygotic) pregnancies there may be one shared placenta, or the babies may have their own placentas.
Where there is one (shared) placenta, this is called a monochorionic pregnancy. There will be one, shared chorion (outer membrane). When there is one chorion, there may be just one amnion (monoamniotic) with all the babies in the same sac of fluid, or there can be more than one amnion, as babies can have their own individual sac of fluid inside the one chorion.
The different types of monozygotic pregnancies are therefore:
- Dichorionic, diamniotic twins. The twins have separate placentas, amnions and chorions. This is true of all non-identical twins, and about a third of identical twins.
- Monochorionic, diamniotic twins. The twins share the same chorion and placenta but they grow in separate amniotic sacs. This is the case for about two thirds of identical twins.
- Monochorionic, monoamniotic twins. The twins share the same amnion, chorion and placenta, so they grow in the same, shared amniotic fluid. This situation is rare, affecting about 1 in 100 identical twin pregnancies.
- Siamese twins (conjoined twins). The fetuses are joined together physically, as the separation of the zygote occurred incompletely, or too late. This 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 two or more babies can share an amniotic sac.
- It is possible to have triplets where two of the babies are identical twins (and may share one placenta, and even one sac) and the third baby is non-identical (with completely separate placenta and sac). This is very rare.
The different types of multiple pregnancy have different risks and possible problems (see below). Pregnancies in which the babies share one placenta have a slightly higher risk of problems.
What types of non-identical twins are there?
Dizygotic twins each have their own placenta, amnion and chorion. The same is true for non-identical triplets. Each fetus develops separately in its own sac with its own blood supply. Although the two placentas can be fused together, each connects to only one baby.
Why does multiple pregnancy occur?
Multiple pregnancy occurs in nature. 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.
Multiple pregnancy is more likely after fertility treatments, particularly assisted reproductive techniques such as in vitro fertilisation (IVF). This is because it is usual for more than one embryo to be transferred to the womb (uterus). In the early days of IVF doctors transferred large numbers of embryos into the womb, as many were lost. However, this resulted in a few women becoming pregnant with very large numbers of babies (in some cases six or more), making the chance of each baby's survival very small and the risks to the health of the mother very large. There is now a strict restriction on the number of embryos put into the womb after fertility treatment.
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 parts of West and Central Africa.
In the UK multiple pregnancy occurs naturally in around 1 out of every 80 births, and identical twins occur only in around 1 in 400 pregnancies. The overall rate of multiple pregnancy in the UK is about 1 in 60 births - because of the extra multiple pregnancies caused by fertility treatment. 9 out of every 10 multiple births are of twins.
The chance of having twins increases if you are older. As you get older, you naturally produce more ovulation-stimulating hormones, which can trigger the ovaries to release several eggs in a single month. The tendency to have non-identical twins also runs in families (passed down the maternal line); however, having identical twins seems to be entirely down to chance.
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 or Central African origin.
- You have had a multiple pregnancy before.
Is antenatal care different in multiple pregnancy?
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.
Ultrasound scans in multiple pregnancy
Multiple pregnancy is usually first recognised at your first ultrasound scan. For most women, this is when you are 11-13 weeks pregnant. Women who have had IVF (and some women who have had bad morning sickness (hyperemesis gravidarum)) will have had an earlier scan and will know sooner.
The 11- to 13-week scan is to check the babies' age, how many babies there are and whether or not they share a placenta. The scan is exactly the same for women with multiple pregnancy as for women with one baby; however, it takes a little longer as there is more than one baby to check. The scan also forms part of the test for Down's syndrome (the other part being a blood test).
After this, the number and frequency of your scans will depend on your individual circumstances. Scans allow the antenatal team to check your babies are growing as they should, and allow early detection of some of the problems twin babies can experience.
Blood tests and medicines
You will have blood tests at least twice in your pregnancy to check you are not becoming anaemic. If you have symptoms of anaemia you may have extra tests. The symptoms of anaemia can be vague and include tiredness, breathlessness and fainting. Anaemia is common in pregnancy and even more common in multiple pregnancy.
Generally speaking, medicines should be avoided in pregnancy unless taken on the advice of a health professional. Women who are at risk of high blood pressure in pregnancy may be advised by their specialist to take a low dose of aspirin. Your specialist will advise if this applies to you.
Down's syndrome testing
Deciding whether to have the test for Down's syndrome is more complicated for multiple pregnancies. The test is less accurate in determining this risk in multiple pregnancies. Your antenatal team will give you information to help you decide whether to have the test.
- If you have identical (monozygotic) twins, the risk of Down's syndrome is the same for each twin.
- If your babies are not monozygotic, the risk of Down's syndrome will be different for each baby.
If you have a higher risk result you will be offered an invasive diagnostic test. About 3-4% of diagnostic tests result in a miscarriage in multiple pregnancy, so this may be a difficult decision. There are other non-invasive DNA tests for Down's syndrome available in the private sector.
The specialist team in multiple pregnancy
You will usually be in the hands of a team of specialists, made up of the ultrasonographer, your midwife and a doctor who specialises in pregnancy and childbirth (an obstetrician). You may also be referred to other specialists for advice if you have symptoms or difficulties.
The highest-risk multiple pregnancies are 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.
Looking after yourself in pregnancy
General advice about diet and lifestyle in pregnancy applies to women who are pregnant with more than one baby. Twin pregnancy is particularly demanding of you, the mother. Many of the 'normal' pregnancy symptoms will come earlier and be more severe. Tiredness, breathlessness and sleep disturbance are very common in the second half of the pregnancy. You may find that your mood goes up and down, that you have strange dreams about labour and birth and that you are much more-than-usually tearful.
When will my twins or triplets be born?
Twins and triplets are on average born earlier than most single babies. On average, twins are born at 37 weeks and triplets at 33 weeks, but there is wide variation in this. See separate leaflet called Premature Labour.
- Sometimes you and your doctors agree that it would be safest for the babies to be delivered early - sometimes very early. Your antenatal team may offer you the option of being induced at 35-37 weeks, or an early planned caesarean section.
- Often, mothers carrying multiple babies go into labour early because their womb (uterus) is being stretched more, and because the space for the babies to grow is limited: twin babies sometimes tend to slow or stop growing towards the end of pregnancy.
The safest option for the babies depends on how many babies you have and whether they share a placenta. 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.
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, and your babies have the best chance of being healthy.
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 and more are almost always delivered by caesarean section.
- Which way around your babies are. If the first baby is 'head down' near the time of delivery, you may usually choose to have a vaginal delivery; however, if both babies are 'breech' you may be advised against it.
- Whether the babies share a placenta or not. If your babies share a placenta you are most often advised to have a caesarean section.
- Whether you have had a caesarean section in the past or not.
- Whether there are any other complications in your pregnancy (for example, high blood pressure, pre-eclampsia, HELLP syndrome, eclampsia).
- Whether the babies become stressed during labour. If the heart rate of either baby slows down, it may become vital to get the babies out quickly.
Your team will discuss the options with you and help 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.
What is a vaginal delivery like with twins?
About 4 in every 10 twin pregnancies result in vaginal delivery. Vaginal delivery with twins is usually not recommended if you have had previous difficulties in labour, a previous caesarean section, if neither of your babies is lying head down, or if your placenta is very low in the womb (uterus). It is also generally not recommended for identical twins who share a placenta (particularly if monoamniotic) or for any triplets (or more).
Your twin labour will be like regular labour in many ways, although you will be closely monitored, including checks on the hearts of two babies, not one. There are usually more doctors and midwives present at a multiple birth, including a separate small team ready to check each baby.
In the case of twin babies there is still only one first stage of pregnancy - the period during which contractions become regular and open up the neck of the womb (cervix). However, there will be two second stages of labour (when you push the baby down).
- In twin labour you will usually be given a drip in case it is needed later - for example, to restart contractions after your first baby is born.
- It's also usually recommended that you have an epidural for pain relief. This makes it easier to deliver your babies quickly if the babies become distressed.
You're more likely to have a vaginal birth if the first twin is in a head-down position (cephalic). The delivery of the first baby will be as for a singleton baby, with you pushing the baby out and your midwife helping control the speed with which your baby arrives. Once the cord is cut the first baby will be handed to their team for a check, and your midwife or doctor will check the position of the second baby by feeling your tummy and doing a vaginal examination. They may also use an ultrasound scan. (The placenta does not come until after the second baby.)
If the second baby is in a good position (breech or cephalic), he or she should be born soon after the first as the cervix is already fully dilated. The time between babies is of the order of minutes and not usually more than 20 minutes (it can be much less). If contractions stop after the first birth, you may be given hormones via a drip to restart them.
As with any delivery, assisted delivery, such as the suction cap (ventouse) or forceps, may be used to help the baby out, particularly if you are finding it difficult to push them out or they are becoming distressed.
In very rare cases, you may deliver the first twin vaginally and then need a caesarean section for the second twin, usually because the second twin is distressed or, after delivery of the first twin, moves into a position which does not allow 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, multiple pregnancy does have a higher risk of problems for both you and your babies. This is why you will be monitored so much more intensively. The risks are higher for babies who share a placenta (monochorionic pregnancies), and higher with higher numbers of babies in the one pregnancy.
What extra risks are there for the mother?
If you are having more than one baby, levels of many pregnancy hormones are higher, and your pregnancy symptoms tend to be worse. Symptoms that tend to be worse include:
- Morning sickness, which is more likely to be severe (called hyperemesis gravidarum).
- Heartburn, mainly because your womb (uterus) is bigger and pushing on your stomach.
- Breathlessness, as your expanding womb presses up on your lungs.
- Problems sleeping, especially as you get bigger.
- Problems associated with more pressure on your blood vessels - for example, piles or varicose veins.
Many other conditions which are common in pregnancy are more common in multiple pregnancy. This does not mean you will develop all these problems, and many women will have straightforward pregnancies. However, the list of things which are more likely to occur with more babies includes:
- Pre-eclampsia - a condition unique to pregnancy in which the blood pressure goes up and protein escapes from your kidneys into your urine - is three times more common in twin pregnancy, and none times more common in triplets. The related conditions of HELLP syndrome and eclampsia are also more common:
- The increased risk of pre-eclampsia 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.
- Gestational diabetes, caused by your body not making/using enough insulin. Symptoms include feeling thirsty, needing to pass urine frequently, and tiredness.
- Anaemia (low levels of haemoglobin in the blood). You will be advised to take iron tablets if you are found to be anaemic.
- Vaginal bleeding. This is often caused by leakage of a little blood from blood vessels on or just inside the neck of your womb (cervix); however, it can come from the placenta and can mean that the placenta is too low or is starting to detach. All types of vaginal bleeding are more common in multiple pregnancy.
- Miscarriage is more common in multiple pregnancy than in singleton pregnancy.
- Obstetric cholestasis occurs when the flow of bile is impaired by pregnancy. The main symptom is severe itching, often on the hands and feet.
- Caesarean section is more common in multiple pregnancy. It may be the advised method of delivery for the safety of you and your babies.
- Polyhydramnios (a condition where there is too much fluid around the baby).
- Excessive bleeding (haemorrhage) just before, during or after delivery.
- Placenta praevia, when the placenta sits a little too low in the womb. The more placenta there is, the more likely it is to encroach on the lower part of the womb, where it may bleed more easily.
- Blood clots (deep vein thrombosis and pulmonary embolism).
- The risk of dying as a result of pregnancy is extremely small in the UK; however, many pregnant women will think about it. The risk is increased if you are carrying more than one baby, but it is still very small, and good antenatal care makes it even smaller.
What extra risks are there for your babies?
In multiple pregnancy there is a higher chance of the following:
- Premature birth. Babies are more likely to be born early, either by natural early onset of labour or because your team thinks it best to deliver them early. Babies born very early have a higher likelihood of problems, both in the short term and in the long term:
- Premature contractions are a common occurrence in pregnancy, particularly with twins and triplets. Most times they are not a sign of preterm labour. However, it can be very difficult to determine if labour is imminent or not and if you experience these symptoms, you should inform a healthcare professional immediately. See separate leaflet called Premature Labour.
- Intrauterine growth restriction (IUGR). Most twins and triplets grow normally in the womb but they tend to be a little smaller than singleton babies, and some are a lot smaller. IUGR is more common in twin pregnancies and even more so in triplets. Premature delivery is sometimes recommended if one or more of your babies is very small and does not seem to be growing much.
- The growth of twins tends to slow down if your pregnancy goes beyond 37 weeks.
- Entangled umbilical cords. This may happen to babies who share a placenta and amniotic sac, particularly during vaginal birth. Sometimes the babies can tangle with one another, preventing you from pushing the first baby out.
- Stillbirth. In pregnancies where there is one baby, the risk of stillbirth is around 5 in every 1,000 births. For twins, this rises to around 12 per 1,000 births, and for triplets 31 per 1,000. The risk is higher where the babies share a placenta.
- Twin-to-twin transfusion syndrome (TTTS): this is described below.
Your antenatal team will be very aware of all these risks; this is why you have extra special care. Together, you and your antenatal team can increase the chance of a safe delivery of healthy babies.
What is twin-to-twin transfusion syndrome?
When you are expecting twins then sometimes one of the babies receives more of the blood supply, usually because the blood supplies from a shared placenta are connected. This condition is called twin-to-twin transfusion syndrome (TTTS).
TTTS happens in about 1 in 8-10 identical twin pregnancies. It is a serious condition, as the twin who is not getting enough blood may become anaemic and may not grow well, whilst the over-supplied twin may be overloaded with fluid.
If you are pregnant with more than one baby, you will have regular scans to watch out for TTTS, particularly if it is thought that your babies share a placenta. If TTTS is picked up you will be monitored very closely and if things seem to be severe enough to affect the babies, there are several possible treatments. The choice varies with the severity of your baby's condition and the stage of your pregnancy. Options include draining some of the amniotic fluid (amnidrainage) from around the twin with the excess supply (which seems to even out the circulation), and in-womb laser treatment of the placenta to seal off blood vessels that are diverting too much blood to one twin. Amnidrainage may need to be repeated, whilst laser treatment is usually one treatment. These procedures are very often successful but there is a risk to your babies and a safer option, if you are further on in your pregnancy, can be to deliver your babies early.
What difficulties can I expect after delivery?
Multiple births are hard work in pregnancy but can also be tough afterwards. Tiredness, baby blues and postnatal depression may all be more marked in mothers of multiple babies.
Babies from multiple pregnancies have a higher chance of needing to go to the special care baby unit (SCBU) or neonatal intensive care (NICU). If you are expecting twins or more you are likely to be offered a tour of the baby units at the hospital where you plan to give birth. It is sensible to do this, as it makes a strange environment seem a little more familiar if you unexpectedly find yourself there.
Feeding and caring for multiple babies is a huge challenge and this won't stop when you leave hospital. You may at times feel completely overwhelmed. In addition to your partner, friends, family, midwife, GP and health visitor there are a number of support groups and organisations which focus on multiple birth and will offer ongoing help and support.
How will I know if my twins are identical?
If your babies share a placenta then you will usually find this out at your first scan, when the ultrasonographer will look closely to discover whether the babies also share one amnion.
If your babies each have their own, separate placenta, then they are most likely to be non-identical. There is a small chance they could be amongst the one third of identical twin pregnancies that have separate placentas. In this case you may not know during pregnancy. After birth it may be difficult to say, just by looking, if your babies are identical. Identical twins can look different at birth. Their heads may be different shapes because they were moulded differently by their position in the womb (uterus) and during labour, and one may be larger than the other.
The DNA test for twins is called zygosity determination. You take DNA samples by sweeping a cotton bud inside each baby's mouth. DNA testing isn't usually available on the NHS but is offered commercially.
Further reading and references
; NICE Clinical Guideline (September 2011)
; Royal College of Obstetricians and Gynaecologists (16th November 2016)
; Planned caesarean section for women with a twin pregnancy. Cochrane Database Syst Rev. 2015 Dec 1912:CD006553. doi: 10.1002/14651858.CD006553.pub3.
; Reduction of the number of fetuses for women with a multiple pregnancy. Cochrane Database Syst Rev. 2015 Nov 411:CD003932. doi: 10.1002/14651858.CD003932.pub3.
; The UK Adult Twin Registry (TwinsUK Resource). Twin Res Hum Genet. 2013 Feb16(1):144-9. doi: 10.1017/thg.2012.89. Epub 2012 Oct 22.
; Twins And Multiple Birth Association (TAMBA).
; Royal College of Obstetricians and Gynaecologists, November 2016
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