How Early Can a Scan Show Twins

When most people actually find out

The NHS says it is usually possible to find out if you are having twins through your 12-week scan. Twins Trust puts it the same way: most people discover they are expecting twins or more at the first ultrasound scan, usually around 12 weeks. That is the honest answer to the question, and it is worth holding on to before reading anything about earlier scans.

What NICE asks that first scan to do

NICE NG137 says women with a twin or triplet pregnancy should be offered a first trimester ultrasound scan to estimate gestational age and determine chorionicity and amnionicity, and adds that ideally these should all be performed at the same scan. The NHS gives the window as around 11 to 14 weeks and calls it the best time to find out what type of placenta and membranes your twins have, and to check your dates.

So the scan is doing three jobs at once: counting the babies, dating the pregnancy, and establishing the arrangement of placentas and sacs. The third job is the one that changes your care.

Dating is taken from the larger baby

NG137 says gestational age should be estimated from the largest baby in a twin or triplet pregnancy, specifically to avoid the risk of estimating it from a baby with early growth pathology. If you are given a single due date for two babies, that is why.

The signs the sonographer is looking for

NG137 says chorionicity and amnionicity should be determined using the number of placental masses, the presence of amniotic membranes and membrane thickness, and the lambda or T-sign. The lambda and T-signs are the shapes the membrane makes where it meets the placenta. NG137 also says sonographers should get regular training to identify them accurately and confidently, which tells you it is not always a glance-and-know judgement.

Can a very early scan settle it?

An early scan may well show two gestational sacs. What it is much less good at is the part that matters. The NHS says that if your first ultrasound scan is done before 14 weeks, it should be possible to tell accurately what kind of placenta your twins have, which places the reliable window inside the first trimester rather than at five or six weeks. NG137's whole schedule is built around a scan performed when crown-rump length measures from 45.0 mm to 84.0 mm, at approximately 11+2 to 14+1 weeks.

There is also a harder truth. Sometimes an early scan shows two sacs and a later scan shows one ongoing pregnancy. The NHS FASP handbook has a formal term for this, a vanished twin, defined as one fetus in a twin pregnancy being non-viable at the dating scan, and it changes which screening tests can be offered. That is a real possibility, not a scanning error.

If you book after 14 weeks

NG137 covers this. If a woman presents after 14+0 weeks, chorionicity and amnionicity should be determined at the earliest opportunity using all of the same features plus discordant fetal sex, because two babies of different sexes cannot be sharing a placenta. It is a useful extra clue that simply is not available earlier.

If the scan cannot tell

NG137 has an escalation path and a safety net. If chorionicity or amnionicity cannot be determined at the time the twin pregnancy is detected, the team should seek a second opinion from a senior sonographer or refer to someone competent in determining it, as soon as possible. If it is still difficult even after referral, for example because of a late booking, the pregnancy should be managed as a monochorionic pregnancy until proven otherwise. In other words, uncertainty is resolved in the direction of more monitoring, not less.

NG137 also says that if transabdominal views are poor, because of a retroverted uterus or a high BMI, a transvaginal scan should be used, and that 3D ultrasound should not be used to determine chorionicity and amnionicity at all.

What this scan does not do

It does not screen for the complications of a shared placenta. NG137 says not to offer women with a twin or triplet pregnancy screening for fetal growth restriction or feto-fetal transfusion syndrome in the first trimester. Those are monitored later, and on a schedule set by chorionicity: NG137 asks for feto-fetal transfusion syndrome monitoring every 14 days from 16 weeks in monochorionic pregnancies, while growth monitoring in dichorionic twins starts from 24 weeks at intervals not exceeding 28 days.

It also does not settle whether same-sex twins with separate placentas are identical. The NHS says the most accurate way to tell is a DNA test after the babies are born.

Naming the babies

One small thing happens at this scan that pays off for the next six months. NG137 says nomenclature should be assigned to the babies, for example upper and lower or left and right, and documented clearly in the notes to ensure consistency throughout pregnancy. Every growth measurement afterwards is compared against the previous one for the same baby, so the labels have to hold.

What to ask before you leave the room

Ask which type of twin pregnancy you have, in the sonographer's own words, and ask for it to be written down. Ask whether the dating has been taken from the larger baby. Ask when your next scan is, because the NHS says DCDA twins are usually scanned every four weeks while monochorionic twins need more. And ask who is now coordinating your care, since NG137 expects a named multidisciplinary team from this point on.