Chorionicity: Why Your Twins' Placenta Type Matters

One of the most important things your doctor will tell you in a twin pregnancy is your chorionicity — whether the babies share a placenta and amniotic sac. It sounds technical, but your entire monitoring plan is built on it. This article is based on the UK national NICE guideline (NG137) and ISUOG (International Society of Ultrasound) recommendations.

There are three types: In DCDA (dichorionic-diamniotic) twins, each baby has its own placenta and its own sac — the most common and lowest-risk type. MCDA (monochorionic-diamniotic) twins share one placenta but have separate sacs. MCMA (monochorionic-monoamniotic) twins share both the placenta and the sac — the rarest type, needing the closest monitoring.

Chorionicity is most reliably determined at the 11-14 week ultrasound: if a "lambda sign" appears where the placenta meets the membrane, separate placentas (dichorionic) are likely; a "T sign" suggests a shared placenta (monochorionic). That's why the first-trimester scan is especially valuable in a multiple pregnancy. If it can't be determined, you may be referred to a specialist center.

Why does monitoring frequency differ? Per the NICE guideline, twins sharing one placenta (monochorionic) are scanned roughly every 2 weeks from 16 weeks — to catch twin-to-twin transfusion syndrome (TTTS) and growth differences early. In twins with separate placentas (dichorionic), growth scans are usually done about every 4 weeks from 20 weeks.

How to use this information: Ask your doctor for your chorionicity and write it down — you can add it to your appointment notes in HeyMom. If you switch hospitals or go to the ER, saying "I have a monochorionic twin pregnancy" lets the team set up the right care plan immediately.

Dear mama, a shared placenta can sound scary, but knowledge is power: with regular monitoring, the vast majority of monochorionic twins are born healthy. Frequent scans aren't an alarm — they're a safety net, and every appointment is one more chance to see your babies.