Twin and multiple pregnancy: chorionicity, scans, twin-to-twin transfusion and birth planning

Twins can come from one fertilised egg that splits or from two eggs, and what matters most for care is what they share. The Royal College of Obstetricians and Gynaecologists describes three arrangements. In dichorionic diamniotic twins, each baby has its own placenta and its own sac. This is the commonest type. In monochorionic diamniotic twins the babies share one placenta but have separate sacs. In the rarer monochorionic monoamniotic type they share both the placenta and the sac.

Chorionicity is best determined on an early scan, because it becomes harder to tell later in pregnancy. It sets the scan frequency, the risks to watch for and the best time to be born.

A woman expecting twins or more should be looked after by a specialist team that includes an obstetrician and a midwife with experience of multiple pregnancy. Shared-placenta twins are scanned at least every 2 weeks from 16 weeks, because problems can develop quickly. Twins with separate placentas have scans at least every 4 weeks from 24 weeks. For triplets, the schedule depends on the details.

Antenatal visits also include blood pressure and urine checks, because pre-eclampsia is more common in multiple pregnancy, and blood tests for anaemia, since two babies increase the demand for iron.

The RCOG explains that carrying more than one baby raises the chance of several problems. Understanding them helps parents spot symptoms early.

• Anaemia from higher iron demands.

• Pre-eclampsia, and the need for early birth if it develops.

• Postpartum haemorrhage, because an overstretched womb may contract less well.

• Operative delivery, by caesarean or instrument.

• Preterm birth, which happens in about 60 in 100 twin pregnancies and 75 in 100 triplet pregnancies.

• Growth problems, including selective fetal growth restriction where one baby grows much more slowly.

In twin-to-twin transfusion syndrome, or TTTS, blood vessels linking the two babies within a shared placenta pass blood unevenly. One baby, the donor, can become small with little fluid around it; the other, the recipient, can have too much blood and too much fluid. According to the RCOG it affects about 15 in 100 monochorionic pregnancies.

This is the reason shared-placenta twins are scanned fortnightly. The scan looks at the amount of fluid around each baby and the bladders. Signs a mother may notice include a tummy that grows quickly over a short time, tightness or discomfort, and sudden breathlessness. Report any of these straight away rather than waiting for the next scan. Diagnosis and treatment are carried out in specialist fetal medicine centres, and the options depend on the stage and how far along the pregnancy is.

Planned birth is brought forward for twins because the risk of stillbirth rises towards term. The RCOG gives these timings: around 37 weeks for DCDA twins, around 36 weeks for MCDA twins, 32 to 34 weeks for MCMA twins and around 35 weeks for triplets. Earlier birth may be needed for pre-eclampsia, poor growth or TTTS, and labour can also start on its own sooner.

Vaginal birth is an option when the first baby is head down and there are no other complications, though more than a third of women planning a vaginal birth need a caesarean. A caesarean is recommended when the first baby is breech and for monoamniotic twins. Whatever the plan, birth takes place in a consultant-led unit with a neonatal team nearby.

Twins and triplets are more likely to need neonatal care because of preterm birth, and parents are commonly exhausted and overwhelmed. Ask early about feeding support, a health visitor with twin experience and local multiple-birth groups. Watch for heavy bleeding after the birth and for low mood.