How do twin pregnancies occur?

Twin pregnancies occur in two main ways:

• Ovulation and fertilisation of two eggs with successful implantation of both.
• Fertilisation of a single egg that goes on to split into two embryos.

The first of these will always produce non-identical twins, even if they are both the same sex. The second will always produce identical twins of the same sex.

Although it always exciting to know whether twins are identical or non-identical, the most important determinant of how your twin pregnancy is managed and the risks involved is how many placentas your babies have – this is known as chorionicity.

Non-identical twins will always have two placentas (dichorionic), even if they are very close to each other, and two amniotic sacs (diamniotic). These type of twins are known as Dichorionic Diamniotic (DCDA).

If a single egg is fertilised and divides early (day 1-3) and produces two placentas and two amniotic sacs, these are DCDA twins (although because they came from the same fertilised egg, they will be identical.)

More commonly, if a single fertilised egg divides, it does so on day 4-8. This produces identical twins with a shared single placenta (monochorionic) and two amniotic sacs (diamniotic). These type of twins are known as Monochorionic Diamniotic (MCDA).

Twins also commonly arise from IVF pregnancies. Even if two embryos were put back, it is possible that, sadly, one did not survive, but the other divided. Similarly, a single embryo may divide.

The optimal time to assess what type of twins you have is by ultrasound scan at 11-14 weeks gestation.

(Other types of twins such as monochorionic monoamniotic (MCMA) and conjoined twins are rare, and care should be individualised by a fetal medicine specialist.)

Screening and prenatal checks

Although some women, especially those following fertility treatments, will be aware from an early stage that they are expecting twins, most women will find out at their routine scan at around 12 weeks gestation.

Screening for Down Syndrome (Trisomy 21), Edward’s Syndrome (Trisomy 18) and Patau’s syndrome (Trisomy 13) can be performed in DCDA twins in the same way as a singleton pregnancy by measuring nuchal translucency (the fluid at the back of the baby’s neck) and the mother’s blood hormone level at 11-14 weeks (the combined test). Individual risks for each baby are calculated. The same screening can be performed in MCDA twins.

Recently, non-invasive prenatal testing (NIPT) has been shown to be a highly accurate screening technique for Down Syndrome. Although the number of tests performed in twin pregnancies is far fewer, there is increasing evidence that NIPT is also accurate in twin pregnancies.

Risks of a multiple pregnancy

Although twin pregnancies are higher risk pregnancies than singleton pregnancies, it is important to remember that a majority of twin pregnancies will progress without any complications.

Twin pregnancies all have an increased risk of the common pregnancy complications. These include gestational diabetes, high blood pressure and preeclampsia. There is a higher chance of growth problems in twins. There is also an increased risk of premature delivery; this may be due to labour spontaneously starting early or due to doctors advising your babies are delivered due to problems.

Twin to Twin transfusion syndrome can only occur in monochorionic twins and affects ~1 in 5 MCDA twin pregnancies. This is a serious condition for your babies in which the twins share part of their blood supply, but this is not equally shared. As a result, one baby (the recipient) gets too much blood and the other baby (the donor) has too little blood (anaemia). TTTS is commonly divided into 5 stages, with stage 1 being least severe, and will require management by a fetal medicine specialist. Stage 1 TTTS can often be managed expectantly with frequent ultrasounds. If TTTS is Stage 2 or higher, a laser procedure to the babies’ placenta is advised (or much less commonly drainage of fluid from around the recipient) to treat the TTTS. This is a highly specialist procedure only performed in a few UK centres and your fetal medicine specialist will talk to you more about this.

Care team for a twin pregnancy

Women with twin pregnancies should be looked after by a team with a special interest in twins. This should consist of, at least, a ‘twins’ midwife and an experienced doctor (obstetrician) with an interest in multiple pregnancies. (More complex twins, such as MCMA, or pregnancies in which complications occur, are likely to be managed by a fetal medicine specialist.)

Due to complications happening more commonly in twin pregnancies, you will have more appointments than a singleton pregnancy. For women with DCDA twin pregnancies, this will include a scan every 4 weeks from 16 weeks gestation. Women with DCDA twins will also have their blood pressure and urine checked every 4 weeks. Care for healthy women with DCDA twins without complications may be mostly midwife-led, although you should meet an obstetrician early in the pregnancy to discuss the local management and later in the pregnancy to make a delivery plan.

Women with MCDA pregnancies should be seen every 2 weeks from 16 weeks including an ultrasound scan. Women with MCDA pregnancies should be aware of how to contact their maternity unit if they experience a sudden increase in abdominal size or sudden shortness of breath.

Management of a twin pregnancy in the later stages

Regular scans will continue. It is normal to feel more tired than in a singleton pregnancy during the last couple of months. Around 50% of twin pregnancies will end with a delivery before 36 weeks gestation. Some of these will be due to complications, but a majority will be due to spontaneous labour between 32-36 weeks gestation. It may be worth bearing these facts in mind when planning maternity leave and work commitments and making arrangements for delivery if it happens earlier than planned.

Choices for delivery

A birth plan should be made with your obstetrician and midwife by around 32 weeks gestation. Delivery will be advised at between 36-37 weeks of MCDA twins and 37-38 weeks for DCDA twins.

In both MCDA and DCDA twin pregnancies that have progressed without complication and in which the first twin is head first (cephalic), vaginal delivery can be considered. A randomised study has shown that there is no difference in outcomes to babies being born vaginally or by caesarean section. There are increased risks of an operation to the mother. In a small number of cases (~5%) of vaginal twin deliveries, an emergency caesarean will be required for the second twin after the first has been born vaginally.

You should discuss your individual care and your maternity unit outcomes with your specialist midwife and obstetrician to plan the best birth option for you and your babies.

Labour, birth and immediate aftercare

If you plan for a natural delivery and labour spontaneously or your waters go, you should contact your maternity unit at the earliest opportunity.

Electronic monitoring of the heart rate, called CTG monitoring, is advised for twin pregnancies once labour is established. The heart rate of both babies will be monitored. Management of the first stage of labour is similar to a singleton pregnancy. If the labour progress is slower than expected, either breaking your waters, starting a hormone drip (Syntocinon) or both may be advised to try and optimise labour progress.

Management of a twin delivery is different from a singleton delivery. The actual delivery of the babies may be performed by a midwife, but there will normally be at least one member of the neonatal team present to check to the babies’ wellbeing immediately after delivery. There is also usually an obstetrician present either in the room or very close by.

The first twin may be delivered normally or by instrumental delivery. Following this, the second twin can change position. The second twin will need its heart rate checking and an ultrasound to check that it is in a suitable position for vaginal delivery. Contractions will normally soon start again or the hormone drip may be started or increased. When the second baby is in a good position and contractions have returned, pushing can start again. In most cases, a vaginal delivery either by maternal pushing or by instrumental delivery will occur for the second twin. However, in a small number of cases an emergency caesarean section is required. You should talk to your midwife and obstetrician in your own maternity unit to discuss your personal plans.

A twin delivery by planned caesarean section is similar to a singleton pregnancy. There are slightly increased risks, especially of increased blood loss. Your obstetrician should discuss these with you when you sign your consent form. In most cases, you’ll be admitted on the day of the caesarean. In many cases, you may be medically fit to go home the next day, although there may be a need to stay in hospital for a few nights if the babies need extra care or help feeding.

If your babies are being born before 36 weeks, it is common that they will need to spend some time in the neonatal unit. In some cases, it may be advised that you are transferred to another hospital to have your babies if there are no neonatal cots available at your hospital. This can be upsetting, but you should be assured that you should only be transferred to another unit that can provide a suitable level of experience and care to both you and your babies.

Main Points

• Non-identical twin pregnancies occur when two eggs are released during ovulation, with successful fertilisation and implantation of both. Identical twin pregnancies occur when a single egg is fertilised which then splits into two embryos.
• The most important determinant of how your twin pregnancy is managed and the risks involved is how many placentas your babies have (chorionicity).
• Non-identical twins will always have two placentas (dichorionic) and two amniotic sacs (diamniotic), known as Dichorionic Diamniotic (DCDA).
• If a single fertilised egg divides, this produces identical twins with a shared single placenta (monochorionic) and two amniotic sacs (diamniotic), known as Monochorionic Diamniotic (MCDA).
• Twins commonly arise from IVF pregnancies.
• Most women will find out that they’re expecting twins at their routine scan at around 12 weeks gestations.
• Non-invasive prenatal testing (NIPT) can be used to test for chromosomal defects in twin pregnancies the same way it can in single pregnancies.
• Although twin pregnancies are at higher risk of problems such as gestational diabetes, pre-eclampsia, and preterm birth, most twin pregnancies progress without any complications.
• Twin to Twin transfusion syndrome (TTTS), a serious condition where the twins share part of their blood supply (meaning one baby gets too much blood and the other gets too little), can only occur in monochorionic twins and affects approximately 1 in 5 MCDA twin pregnancies.
• If TTTS is severe, a highly specialist laser procedure to the babies’ placenta is advised.
• Women with twin pregnancies should be looked after by a team with a special interest in twins. Complex twins and complicated pregnancies are likely to be managed by a fetal medicine specialist.
• Women with twin pregnancies are more likely to have more appointments and scans than those with a singleton pregnancy.
• Management of a twin delivery is different from a singleton delivery. There will normally be at least one member of the neonatal team present other than a midwife to check to the babies’ wellbeing after delivery.
• The second twin may change position following the normal or instrumental delivery of the first. An ultrasound will be used to check whether they are in a suitable position for vaginal delivery. In most cases, vaginal or instrumental delivery is possible for the second twin, but an emergency caesarean may be required in some cases.
• A planned caesarean section for a multiple pregnancy will be similar to a caesarean in a singleton pregnancy, but there are a few increased risks, including increased blood loss.
• Babies born before 36 weeks may need to be admitted to the neonatal unit, which may mean they need to be transferred to another hospital.