Twin pregnancy scans: chorionicity, TTTS and how often to scan

Medically written by Dr. Sushma Dhakal, MBBS · MD (Obstetrics & Gynaecology) · Fellowship in Fetal Medicine · Updated 30 October 2026 · 3 min read

Expecting twins is exciting, and it also means closer monitoring. The single most important fact about a twin pregnancy is its chorionicity: whether the babies share a placenta. This decides how often you are scanned and which complications to look for.

Types of twins

Dichorionic diamniotic (DCDA): each baby has its own placenta and sac. About two thirds of twins. All non-identical and some identical twins are DCDA.
Monochorionic diamniotic (MCDA): the babies share one placenta but have separate sacs. Always identical twins.
Monochorionic monoamniotic (MCMA): one placenta and one sac. Rare and needs very specialised care.

When is chorionicity seen?

It is most accurately determined at the 11–14 week scan, or earlier. A thick, triangular "lambda" sign where the membrane meets the placenta suggests DCDA twins; a thin "T" sign suggests monochorionic twins. Later in pregnancy this becomes much harder to tell, so an early scan is very valuable. The NT scan is also used to screen each baby individually and to label the twins consistently.

How often are twins scanned?

DCDA twins: an anomaly scan at about 20 weeks, then growth scans about every 4 weeks.
Monochorionic twins: scans about every 2 weeks from 16 weeks, checking fluid around each baby, bladders, Doppler flows and growth, plus a detailed heart scan.

Twin-to-twin transfusion syndrome (TTTS)

Because monochorionic twins share blood vessels in the placenta, blood can flow unevenly from one twin to the other. TTTS affects about 10–15% of monochorionic twins, most often between 16 and 26 weeks. One twin (the donor) has too little fluid and a small or empty bladder; the other (the recipient) has too much fluid. It can progress quickly, which is why two-weekly scans matter.

Severe TTTS is usually treated with fetoscopic laser surgery at a specialist centre, which seals the connecting vessels. Early detection gives the best chance of timely referral.

Please seek care promptly if your tummy suddenly gets much bigger or tighter, you feel breathless, or you have pain or leaking fluid.

Other complications to watch

Selective growth restriction: one baby grows much more slowly than the other.
TAPS: a slow imbalance of red blood cells between monochorionic twins, detected by brain Doppler.
Higher chances of preterm birth, pre-eclampsia and anaemia in all twin pregnancies, so regular antenatal checks, iron and blood pressure monitoring are important.

Delivery

Most uncomplicated DCDA twins are planned for birth around 37 weeks and MCDA twins around 36 weeks. The method of birth depends on the babies' positions and your history, and will be discussed with you in advance.

Have questions about your pregnancy?

Book a consultation with Dr. Sushma Dhakal, Fellowship-trained fetal medicine specialist (FMF UK certified).

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Frequently asked questions

When is the best time to know if twins share a placenta?

At or before the 11–14 week scan, when chorionicity can be seen most accurately.

How often are monochorionic twins scanned?

About every two weeks from 16 weeks, to detect TTTS and other complications early.

Can TTTS be treated?

Yes. Severe TTTS is usually treated with fetoscopic laser surgery at a specialist centre.

Are all identical twins monochorionic?

No. About one third of identical twins have separate placentas (DCDA).

This guide is for general information only and is not a substitute for personal medical advice. Every pregnancy is different. Please consult your doctor about your own situation. In an emergency, go to the nearest hospital.