Understanding Your Twin Pregnancy: Chorionicity and Why Your Scan Schedule Is Different

By Jennifer James, UK twin mum and author of Outnumbered From Day One, the honest guide to life with twins.

Somewhere around your first scan, a sonographer will say a word that decides almost everything about the rest of your pregnancy: chorionicity. Nobody explains it properly at the time. Here's the explanation you should have got.

Just found out it's twins? Start with our guide to expecting twins in the UK.

Identical and non-identical isn't the useful distinction

Everyone asks whether your twins are identical. The more useful question, medically, is whether they share a placenta. That's chorionicity, and it's what your care team actually plans around.

• Dichorionic (DC): each baby has their own placenta. This is always the case for non-identical twins, and true for around a third of identical twins too, depending on when the egg split.
• Monochorionic (MC): both babies share one placenta. This only happens with identical twins, and it's the reason identical twin pregnancies are monitored more closely than non-identical ones.

Chorionicity is usually confirmed at your dating scan, around 11 to 14 weeks. It's the single most important thing your team establishes early, because it's genuinely harder to determine later on. (NICE guideline NG137)

Why monochorionic pregnancies mean more scans

If your babies share a placenta, blood vessels connect the two circulations. Most of the time that's fine. Sometimes the flow between them becomes uneven, and that's what your extra scans are watching for. It's why monochorionic pregnancies are typically scanned every two weeks from 16 weeks, rather than the standard schedule. It isn't because something's wrong. It's because something could develop, and catching it early makes an enormous difference.

What TTTS actually is

Twin to twin transfusion syndrome (TTTS) happens when the blood flow between two babies sharing a placenta becomes unbalanced, one twin gets too little, the other gets too much. Around 10 to 15% of monochorionic pregnancies develop it, most often diagnosed before 24 weeks. (Twins Trust on TTTS) Regular scans are how it's picked up, almost always before you'd notice anything yourself. This is the whole reason for the tighter monitoring schedule, and it's genuinely effective. Turning up to every scan you're offered is one of the most protective things you can do.

Keeping track of what happens when

Between dating scans, growth scans, doppler checks and consultant appointments, it gets hard to hold in your head, especially if you're also managing morning sickness, a job, or another child. That's the whole reason we built a tracker for it: one place for every appointment, every result, and the questions you meant to ask and forgot.

If you want the fuller picture of what the first trimester actually feels like, not just the clinical side, read the first trimester post, or grab the free sample chapter from the book.

FAQ

What's the difference between identical and non-identical twins in terms of chorionicity? Identical twins can be either monochorionic (sharing one placenta) or dichorionic (each with their own), depending on when the egg split. Non-identical twins are always dichorionic, since they come from two separate eggs.

When is chorionicity determined? Usually at the dating scan, around 11 to 14 weeks. It's harder to determine accurately later on, which is why this scan matters so much.

How often will I be scanned if I'm monochorionic? Typically every two weeks from 16 weeks, compared with a less frequent schedule for dichorionic pregnancies, because of the closer monitoring needed for shared-placenta risks like TTTS.

What percentage of monochorionic pregnancies develop TTTS? Around 10 to 15%, according to Twins Trust, usually diagnosed before 24 weeks through routine scans rather than symptoms you'd notice yourself.

Keep every appointment straight with the Twin Pregnancy Appointment & Scan Tracker.