Multiple pregnancies are getting more common because they are linked to infertility treatments, which are becoming more available. The most common multiple is twins.
Twins can be monozygotic (one fertilised egg splits in two) or dizygotic (two eggs are fertilised). Monozygotic twins will normally be the same sex. Infertility treatments are linked to dizygotic twins.
Most women who are pregnant with twins deliver healthy babies but carrying more than one baby increases the health risks for the mother and baby.
There are three types of twins. These apply to triplets too, although a triplet pregnancy will be more complex than a twin one. The three types are:
- dichorionic diamniotic (DCDA) twins – each has their own separate placenta with its own separate inner membrane (amnion) and outer membrane (chorion)
- monochorionic diamniotic (MCDA) twins – share a single placenta with a single outer membrane and two inner membranes
- monochorionic monoamniotic (MCMA) twins – share both the inner and outer membranes
All non-identical twins are DCDA, and one-third of identical twins are DCDA. The other two-thirds of identical twins are MCDA, and just 1% of identical twins are MCMA.
One-third of all twins will be identical and two-thirds non-identical
- Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in two, creating identical twins who share the same genes. Identical twins are always the same sex and they'll look very alike.
- Non-identical (dizygotic) twins happen when two separate eggs are fertilised and then implant into the womb. These non-identical twins are no more alike than any other two siblings. The babies may be the same or different sexes.
Multiple pregnancy is normally spotted in routine pregnancy scans and most women know by 20 weeks that they are carrying more than one baby.
Carrying more than one baby means you are more likely to have a premature delivery.
The average gestation for multiples is:
- Twins: 37 weeks
- Triplets: 34 weeks
- Quadruplets: 32 weeks
If the baby is not born by then, most twin pregnancies are induced by 38 weeks because the health risks increase after that.
What are the risks and complications of multiple pregnancy
Most women with twins stay healthy through the pregnancy and deliver healthy babies. However, you will be at higher risk of some of the potential complications that can cause premature birth or lead to induction, including:
- antepartum and postpartum haemorrhage (severe vaginal bleeding)
- polyhydramnios (too much amniotic fluid)
- placental abruption
- hyperemesis gravidarum
- intrauterine growth restriction (baby not growing properly)
- gestational diabetes
- needing an assisted delivery with forceps, ventouse or caesarean section
- a problem called twin to twin transfusion syndrome (see below) if the babies share the same placenta.
What treatment can I expect if I have a multiple pregnancy?
Multiple pregnancies are considered high risk so you will be put into the care of a special team of obstetricians and specialist midwives who will monitor your pregnancy and baby carefully for the problems above.
The delivery, vaginal or caesarean, will depend on the position of your babies. There will be a specialist team present at the birth in case of complications. It will include midwives, obstetricians, anaesthetists and paediatricians.
Twin to twin transfusion syndrome (TTTS)
Twin-to-twin transfusion syndrome (TTTS) can affect identical twins who share a placenta (monochorionic twins).
Twin to twin transfusion syndrome is an imbalance in the placental blood vessels that connect the twins.
The blood doesn't flow evenly and one gets more blood (called the recipient) and the other gets less (the donor).
Twin to twin transfusion syndrome is not common with around 10-15% of monochorionic twins affected. The majority of twins who share a placenta grow normally.
The recipient twin gets too big because they are getting extra nutrients and fluid. The donor twin grows too slowly.
Although it seems like the recipient twin is benefiting, this can affect the health of both twins. The extra fluid the recipient gets can put a strain on their heart. They will try to get rid of it by producing more urine, which means they may have too much amniotic fluid around them, while the donor twin will have little or none.
If your twins share a placenta, you should have regular ultrasound scans to look for early signs of TTTS. The scans will take place every two to three weeks from 16-24 weeks and onwards. If there are signs that TTTS may be developing, you should have weekly scans and be referred for treatment at a fetal medicine specialist centre.
If you have a monochrionic twin pregnancy you will also be asked to watch out for a sudden increase in size or a feeling of breathlessness as these are possible symptoms of twin to twin transfusion syndrome.
Treating twin to twin transfusion syndrome
If the TTTS is mild, you may require no treatment and it is possible that the condition will remain stable or improve.
Your pregnancy will be closely monitored and if TTTS becomes more severe, your doctors may recommend intervention, such as laser surgery or amnioreduction (draining excess amniotic fluid from around the larger recipient twin).
Your doctor may recommend a planned caesarean for your twins at 34 weeks, or earlier if your smaller twin (donor) does not catch up after the treatment. Babies affected by TTTS are more likely to be born prematurely.
The specialist centre where you have your care will have access to a neonatal intensive care unit (NICU), in case your babies need treatment as soon as they're born. At least one of your twins is likely to need to spend time in the NICU. It's worth talking to a neonatologist about what to expect, and having a tour of the NICU while you're pregnant. You can also read our information about giving birth to a premature baby here.
- Macdonald S (2012) Mayes Midwifery, Fourteenth Edition, Balliere Tindall Elseviere
- Norman J, Greer I (2011) Preterm labour, managing risk in clinical practice, Cambridge University Press
- NICE (2011) CG129 Multiple pregnancy: full guideline, National Institute of Health and Clinical Excellence
ℹLast reviewed on October 5th, 2016. Next review date October 5th, 2019.