Twin pregnancy: what to expect from diagnosis to birth
A multiple pregnancy brings a different antenatal journey from the moment it is confirmed. Around 1 in 65 pregnancies in the UK is a multiple pregnancy, and although twins are by far the most common type, the antenatal care, monitoring, and decisions about birth are shaped by factors that do not apply to singleton pregnancies. The two most important of these factors are the type of twins and, in particular, whether they share a placenta. Understanding this early in your pregnancy allows you and your maternity team to put the right level of monitoring in place from the outset.
How twins form: the two main types
Twins are classified in two broad ways. Non-identical twins, sometimes called dizygotic twins, develop when two separate eggs are fertilised by two separate sperm. Each twin has its own placenta and its own amniotic sac. This arrangement is called dichorionic diamniotic, or DCDA, and it carries the fewest additional risks associated with sharing resources before birth.
Identical twins, or monozygotic twins, develop when a single fertilised egg splits early in development. Depending on when the split occurs, the twins may share a placenta (monochorionic) while still having separate amniotic sacs (diamniotic, making them MCDA), or in rarer cases they may share both a placenta and a sac (MCMA). MCDA twins are the most common type of identical twins, and they require more intensive monitoring than DCDA twins because sharing a placenta introduces a set of risks that do not exist when each baby has its own separate blood supply.
Chorionicity, meaning whether the twins share a placenta, is best determined by ultrasound in the first trimester, ideally before 14 weeks. The features visible on an early scan are the most reliable way to make this distinction, and getting an accurate picture at this stage has a direct influence on the care plan for the rest of the pregnancy.
Risks specific to twin pregnancy
Carrying more than one baby increases the likelihood of several complications, regardless of the type of twins. Preterm birth is among the most common: twin pregnancies are more likely to end before 37 weeks, and both babies may need time in a neonatal unit after delivery. Gestational diabetes, which involves raised blood sugar during pregnancy, is more common in multiple pregnancies, as is pre-eclampsia, the condition involving high blood pressure and protein in the urine. Fetal growth restriction, where one or both babies do not grow as expected, is also a recognised risk that your team will monitor for through regular growth scans.
For MCDA twins specifically, sharing a single placenta creates the possibility of an uneven distribution of blood flow between the two babies. This can lead to twin-to-twin transfusion syndrome (TTTS), a serious condition in which one twin receives too much blood and the other too little. TTTS can develop at any point during the pregnancy and requires urgent specialist assessment if suspected. The more frequent scanning schedule for monochorionic pregnancies exists partly to catch the early signs of this condition as soon as possible.
Antenatal care: a more intensive schedule
The antenatal pathway for a twin pregnancy involves more appointments and more scans than a singleton pregnancy. For DCDA twins, the NHS recommends scans every four weeks from 20 weeks of pregnancy onwards, with regular checks on growth, blood flow, and amniotic fluid levels. Appointments are also more frequent to allow blood pressure and urine to be monitored for signs of pre-eclampsia and gestational diabetes.
For MCDA twins, the monitoring is more intensive still. Scans are recommended every two weeks from 16 weeks onwards. This frequency reflects the higher risk of complications associated with sharing a placenta, and it means that if twin-to-twin transfusion syndrome or another problem begins to develop, there is a much smaller window between detection and the point at which the last scan was taken. Your care will be led by a consultant obstetrician rather than midwife-led care alone, and you will typically be seen at a hospital that has experience managing monochorionic twin pregnancies.
Planning for birth
Twin pregnancies are generally delivered earlier than singleton pregnancies. Delivery before 37 weeks is often recommended to reduce the risks associated with continuing the pregnancy to full term, and the timing will depend on the type of twins and how the pregnancy is progressing. For non-identical twins with separate placentas and no complications, delivery around 37 weeks is commonly advised. For identical twins sharing a placenta, delivery is usually recommended between 36 and 37 weeks.
The mode of delivery depends on a number of factors, the most important of which is the position of the first twin, meaning the baby closest to the birth canal. If the presenting twin is head-down, a vaginal birth is possible, and many people expecting twins do give birth vaginally. Your consultant will discuss the evidence around vaginal birth versus caesarean section with you, taking into account your individual circumstances. If a vaginal birth is planned, it will take place in a consultant-led unit with theatre immediately available, because the position of the second twin can sometimes change after the first is born and a caesarean may become necessary.
If the first twin is not in a head-down position, or if there are other clinical reasons, a planned caesarean section will be recommended. Your maternity team will explain the reasons for any recommendation they make, and you will have the opportunity to ask questions and discuss your preferences before a plan is agreed.
After the birth: neonatal care
Because twins are more likely to be born early, admission to a neonatal unit is more common than for singleton births. The level of care needed depends on how early the babies are born and how well each baby is doing at delivery. A neonatal unit is staffed by specialist nurses and doctors and provides the monitoring, warmth, and feeding support that early babies often need. Your maternity team will prepare you for this possibility during your antenatal appointments so that it is not a surprise if it happens.
If your babies do go to the neonatal unit, you will still be encouraged to have as much contact with them as possible, including kangaroo care (skin-to-skin) and feeding support from the unit's staff. Most twins who need a short neonatal stay go home without any long-term complications.
Frequently asked questions
What is the difference between identical and non-identical twins?
Non-identical twins develop from two separate fertilised eggs and each has their own placenta. Identical twins come from one fertilised egg that splits. Identical twins may share a placenta, which requires closer monitoring as it carries additional risks.
Why does it matter whether twins share a placenta?
Twins who share a placenta have a higher risk of complications including twin-to-twin transfusion syndrome (TTTS). This is why monochorionic twins need more frequent scans and closer monitoring throughout pregnancy.
When will I give birth if I am expecting twins?
Twin pregnancies are typically delivered before 38 weeks. For non-identical twins with separate placentas, delivery around 37 weeks is common. For identical twins sharing a placenta, delivery is usually recommended at 36 to 37 weeks.
Can I have a vaginal birth with twins?
A vaginal birth is possible if the presenting twin is in a head-down position. Your consultant will discuss the evidence and options with you. It will take place in a consultant-led unit with theatre immediately available.
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