Types of Twins and Chorionicity

The question everyone asks, and the one that matters

When people find out you are expecting twins, they ask whether the babies are identical. When your maternity team finds out, they ask something different: how many placentas, and how many sacs. NICE puts the reason bluntly in its training recommendation for sonographers, saying they should be taught that "the risks associated with twin and triplet pregnancy are determined by chorionicity and not zygosity". Zygosity is whether the babies came from one egg or two. Chorionicity is how the pregnancy is plumbed. Only the second one changes your care.

The vocabulary is worth learning early, because it will be used at every appointment from the first scan onwards, usually without anyone stopping to define it.

Identical and non-identical, briefly

The NHS describes identical (monozygotic) twins as the result of a single fertilised egg that divides in two, so the babies share the same genes and are always the same sex. Non-identical (dizygotic) twins come from two separate eggs fertilised by two separate sperm, and the NHS notes they are "no more alike than any other 2 siblings" and may be the same sex or different sexes. On the NHS's figures, roughly one-third of all twins are identical and two-thirds are non-identical.

The NHS also records that everyone has about a 1 in 250 chance of having identical twins, and that identical twins do not run in families. Non-identical twins do, on the mother's side, because the tendency to release more than one egg can be inherited. Tommy's, whose multiple pregnancy information is clinically reviewed, adds that around 1 in every 60 births in the UK are twins, triplets or more.

The three types of twin pregnancy

The NHS sets out three types, and says they apply to triplets too, although a triplet pregnancy is more complex.

Dichorionic diamniotic (DCDA)

Each baby has a separate placenta and a separate sac. The NHS says all non-identical twins are DCDA, and that some identical twins are too. This is the arrangement with the lowest risk of the complications specific to multiple pregnancy, and the NHS says DCDA twins are usually scanned every four weeks.

Monochorionic diamniotic (MCDA)

The babies share a single placenta but have their own sacs. The NHS says most identical twins are MCDA, and that this group "has the highest risk of twin-twin transfusion syndrome (TTTS)". More scans and closer monitoring follow automatically.

Monochorionic monoamniotic (MCMA)

The babies share both a placenta and a sac. The NHS calls this "a much rarer type of pregnancy" and notes that "there's often some cord entanglement, which can cause complications". RCOG's Green-top Guideline No. 51 puts monochorionic monoamniotic pregnancies at about 1% of twin pregnancies and describes them as carrying "a very high risk of cord entanglement".

Tommy's gives the rough distribution across twin pairs as around 70 to 75 in 100 DCDA, 20 to 30 in 100 MCDA, and 1 or 2 pairs in 100 MCMA. RCOG states that approximately 30% of twin pregnancies in the UK are monochorionic.

Why timing decides the arrangement

For identical twins, the type depends on when the fertilised egg split. Tommy's explains that a split in the first three days usually gives each baby their own placenta and sac, which happens in about 20 to 30 out of 100 identical twins. A split between days four and eight usually produces one shared placenta with separate sacs, and Tommy's calls this the most common pattern, at 60 to 70 out of 100 identical twins. It also notes that twins cannot share a sac without also sharing a placenta.

How the scan works it out

NICE recommends a first trimester ultrasound scan to estimate gestational age and determine chorionicity and amnionicity, ideally all at the same scan. The sonographer uses three things: the number of placental masses, the presence and thickness of the amniotic membranes, and what NICE calls "the lambda or T-sign" — the shape where the membrane meets the placenta. The NHS advises that multiple pregnancies should be offered a scan at around 11 to 14 weeks, and says this "is the best time to find out what type of placenta and membranes your twins have".

If you book later, NICE says chorionicity should be determined at the earliest opportunity using those same three features plus discordant fetal sex. Different sexes settle it: the pregnancy is dichorionic.

NICE also tells services what to do when the picture is unclear. A second opinion from a senior sonographer should be sought, or you should be referred to someone competent in determining chorionicity. If a transabdominal view is poor because of a retroverted uterus or a higher BMI, NICE says to use a transvaginal scan. It specifically says not to use 3D ultrasound for this. The NHS adds that if it is not clear from the first scan, you should be offered another one.

What happens when it still cannot be established

NICE has a safety net, and it is worth knowing about because it can feel alarming if nobody explains it. If chorionicity is still difficult to determine even after referral — for example because you booked late — the guideline says to "manage the pregnancy as a monochorionic pregnancy until proven otherwise". You are not being told your babies share a placenta. You are being given the more intensive monitoring pathway because it is the safer default.

Naming the babies, and keeping the names

NICE asks teams to assign nomenclature to each baby — upper and lower, or left and right — and to document it clearly so the labels stay consistent throughout the pregnancy. This sounds administrative, but it is what makes growth charts meaningful. If baby one becomes baby two between appointments, nobody can tell whether a difference in size is real or a labelling error.

Finding out for certain, later

Same-sex DCDA twins are the ambiguous case. Tommy's notes that when twins are the same sex and each has their own placenta and sac, it can be hard to know whether they are identical or non-identical. Both the NHS and Tommy's say the most accurate way to tell is a DNA test after birth. Twins Trust describes this as a zygosity test using painless cheek swabs from each twin, which can be done at any age.

None of this changes the pregnancy you have already had. It answers a question about your children, not about your care.

The one thing to take away

Ask at your first scan which of the three types you have, and write the answer down. Every recommendation that follows — how often you are scanned, which complications your team is watching for, when your babies are likely to be born — hangs on that one word.