When a scan shows two heartbeats, the most important thing to find out next is not whether the twins are identical, but whether they share a placenta. That is called chorionicity, and it is most accurately seen on a scan before 14 weeks.
The three main types
- Dichorionic (DCDA): each twin has its own placenta and its own sac. This is the most common and lowest-risk type.
- Monochorionic diamniotic (MCDA): the twins share one placenta but have separate sacs. Always identical.
- Monochorionic monoamniotic (MCMA): one placenta and one sac. Rare, and needs very close monitoring.
Why sharing a placenta matters
In monochorionic twins, blood vessels in the shared placenta connect the two babies’ circulations. Usually the flow is balanced. In about one in ten such pregnancies it becomes unbalanced - twin-to-twin transfusion syndrome (TTTS) - and one twin receives too much while the other receives too little. Picked up early, TTTS can be treated, which is why these twins are scanned every two weeks from 16 weeks.
What your scan schedule looks like
- DCDA twins: growth scans about every four weeks from 20 weeks
- MCDA twins: scans every two weeks from 16 weeks, checking fluid, bladders and growth
- All twins: a detailed anomaly scan, and cervical length checks for preterm risk
When are twins born?
Uncomplicated dichorionic twins are generally delivered around 37 weeks, and monochorionic diamniotic twins around 36 weeks. Whether a normal delivery is suitable depends mostly on the position of the first twin. The plan is discussed with you well before the due date, with the neonatal team informed in advance.
Related care: Twin and multiple pregnancy care
More patient guides
- The NT scan at 11–14 weeks: what it checks and how to prepare
- High blood pressure in pregnancy: the warning signs you should not wait on
- NIPT or NT scan: which test do you need, and can one replace the other?
- Pregnancy after 35: fertility, risks and how to plan a healthy pregnancy
- Uterine fibroids: symptoms, when to treat, and the options available
This guide is general information for patients and does not replace a consultation with your own doctor.