Multiple pregnancy

Multiple pregnancy means carrying more than one baby, normally twins.

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.

If you're expecting twins or more, join BabyCentre's friendly multiple pregnancy support group to chat and swap tips with others in the same situation.

Premature birth

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:

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.


  1. Macdonald S (2012) Mayes Midwifery, Fourteenth Edition, Balliere Tindall Elseviere
  2. Norman J, Greer I (2011) Preterm labour, managing risk in clinical practice, Cambridge University Press
  3. NICE (2011) CG129 Multiple pregnancy: full guideline, National Institute of Health and Clinical Excellence


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    Last reviewed on October 5th, 2016. Next review date October 5th, 2019.

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    Please note that these comments are monitored but not answered by Tommy’s. Please call your GP or maternity unit if you have concerns about your health or your baby’s health.
    • By Michael Ray Overby (not verified) on 2 Aug 2017 - 02:38

      Hello to Tommy's Midwives...
      I & 17 women run MoDi Twins & MoDi Twins Pregnancy, support groups on Facebook for women involved in MCDA gestation. It's great to see you give mention to the #1 killer of twins In Utero. We deeply appreciate that. Let me tell you a couple things that aren't readily apparent, or possibly just don't get discussed.

      First let's talk about that vessel imbalance. That is, in point of fact, what is Behind TTTS , but in a rather Markedly different way than may be Portrayed.

      TTTS is, Firstly, a disorder of Runaway Growth of the placentation. One may believe that nutrient rich blood is Stolen by Recipient Twin & expropriated for her own use (Evil Twin!), but this is not at all what is happening. A little discussion on the placenta itself thus becomes necessary... For a Long Time, many believed that Placenta Grew Baby under it's Direction. Placenta would Size Up & tell baby to Get Bigger, using Growth Hormones. This theory works Fine and Dandy,& may have gained Full Acceptance if it weren't for All those Pesky MoDi... Twins, with a Shared Placenta... When we look at those, all kinds of Inconveniences pop up.
      Placenta is known now to be a fast responding organ. Baby senses Low Fluid, her Adrenal Complex begins to Shed VEGFR-ES, a growth hormone for the placenta that is a request for More Fluid from it.
      Works Beautifully, & in a Most Elegant Fashion.
      For Singletons.
      Consider now please Two Babies, plumbed into a Shared Placenta, with Shared Circulation across said Placentation. Consider a Pair that is "Growing Normally", of nearly equal Size & Weight. Each baby signals for placental growth as she sees a Water Shortage. No Problem.

      THEN, a vessel grows across the placenta, sending more fluid to one twin, without
      a Corresponding one growing in the opposite direction.
      What happens then?
      The twin that is losing fluid, becomes the Donor. Due to the Transfusion happening from that vessel growing.
      No Problem, she signals for placental growth.
      Problem: Her side of the Placentation is Dried Out. It tries to make more fluid but cannot grow very well.
      Additional Problem: Recipient's side of the placentation is Ideal for growth, & much more fluid capacity is engaged, which makes a whole lot more fluid available for Recip Twin, that she Does Not Need. That she Did Not Steal either.
      If this state continues & no blood vessel to balance the state of the placental flow balance grows, or unless Shred Circulation is Curtailed by Laser, TTTS will continue until one or both twins have died.

    • By Midwife @Tommys on 2 Aug 2017 - 09:47

      Hi Michael, thank you for your comment and support. We really appreciate you sharing your knowledge for our Tommy's followers/supporters/families.
      All the best :)

    • By Natalie Gilbert (not verified) on 2 Feb 2017 - 23:00

      I too went through the horror of being diagnosed with ttts. It was at 16 weeks. One baby had more fluid around him than the other. I had regular scans every week. Then at 19 weeks I went from stage 1 to stage 3 in one week and had to have laser surgery to which I felt everything. They burst a blood vessel going on and said they would have to kill one of my babies. It was devastating. Luckily they stopped the bleed and things resumed back to normal. All fluids the same and blood was the same to both baby boys. To my horror my waters broke at 23 weeks. I was transferred to the Rosie hospital in Cambridge. And all the doctors gave them no hope in hell in surviving. Gave me an approx weight for both. At 24 weeks I gave birth naturally. Charlie the smallest twin died in my arms whilst i was still giving birth to his brother. Alfie died shortly after although he was quite bigger. The doctors did everything they could. I'm not sure the boys were ready for this cruel world and wanted to be together. The loss of them still dooms me to this day and apart of me went down with them when i buried them. They will always be mummies angels. And they live on in my two boys that i have here. They love there brothers and there's never a day that goes by that we don't think of them.

    • By Midwife @Tommys on 3 Feb 2017 - 10:28

      I am so terribly sorry to hear what you have been through with the loss of your two boys Charlie and Alfie, I cannot even begin to imagine how this has been for you. If we can support you in anyway then please do email us in confidence, [email protected] or call our midwife line on 0800 0147 800. Take care, Tommy's midwives.

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