Twins & multiples

The question that shapes the whole pregnancy

With twins, the thing that matters most is not whether they are identical. It is whether they share a placenta — and that single fact determines how closely the pregnancy is watched, how often you are scanned, and when birth is likely to be planned.

Twins with their own placenta and their own sac are the most common arrangement and the most straightforward. Twins sharing one placenta but with separate sacs need considerably closer monitoring. Twins sharing both a placenta and a sac are rare and are managed as high risk throughout. The terms you will see written down are dichorionic diamniotic, monochorionic diamniotic and monochorionic monoamniotic, usually shortened on notes to di/di, mono/di and mono/mono.

This is established most reliably on a scan in the first trimester, by the shape of the membrane where it meets the placenta. It gets harder to determine as the pregnancy goes on, which is the practical reason an early scan matters more with twins than with one baby. If nobody has told you which arrangement yours is, it is a fair thing to ask directly.

Where a placenta is shared, the circulations are connected, and the risk is that blood flow between the two becomes unequal. That is why monochorionic twins are typically scanned every two weeks from the middle of the second trimester rather than at the usual intervals — the condition develops faster than a monthly appointment would catch, and is treatable when it is found early.

Identical and fraternal are a separate question about how the pregnancy began, and the two do not map neatly onto each other. Fraternal twins always have their own placenta; identical twins may or may not, depending on how early the split happened. It is a common source of confusion, and it is why your notes talk about chorionicity rather than about whether the babies will look alike.