Twin Pregnancy: Chorionicity, Scans & Delivery

A twin pregnancy is not simply a pregnancy with an extra baby. It is a different clinical situation, with its own scan schedule, its own complications, its own delivery timing and its own risks — and almost all of it turns on one piece of information gathered in the first trimester.

That information is chorionicity: whether the babies have one placenta between them or one each. It is easiest to determine early, becomes considerably harder after 14 weeks, and it decides how often you are scanned, what can go wrong, and when your babies are born.

This page walks through the whole of it, in the order a twin pregnancy actually unfolds.

Chorionicity: the question that decides everything

Two things are described when twins are found: how many placentas, and how many sacs.

• Dichorionic diamniotic (DCDA) — two placentas, two sacs. The commonest arrangement, and the lowest-risk. All non-identical twins are DCDA, and about a third of identical twins are too.
• Monochorionic diamniotic (MCDA) — one shared placenta, two sacs. Always identical. This is the group that needs intensive surveillance.
• Monochorionic monoamniotic (MCMA) — one placenta, one sac, both babies in the same space with no membrane between them. Rare, and the highest-risk arrangement, because the cords can tangle. These pregnancies belong in a tertiary fetal medicine centre from the start.

The distinction that matters clinically is not identical versus non-identical — it is shared placenta versus separate placentas. A shared placenta means shared blood vessels, and shared blood vessels are what create the complications unique to twins.

How it is determined

On the first trimester scan, by looking at three things: how many placental masses there are, how thick the dividing membrane is, and the shape of the membrane where it meets the placenta. A thick triangular wedge of placental tissue pushing into the base of the membrane — the lambda sign — means two placentas. A thin membrane meeting the placenta at a right angle, the T-sign, means one shared placenta.

After 14 weeks these signs become progressively less reliable, and if the babies are the same sex it may not be possible to be certain at all. This is the single strongest argument for having the dating scan on time in a twin pregnancy.

One technical point worth knowing: in twins, gestational age is estimated from the larger baby, so that an early growth problem in one does not make the whole pregnancy appear younger than it is.

How often you will be scanned

This is where chorionicity translates into practice, and the difference is stark.

If the babies have separate placentas (DCDA)

At least eight antenatal appointments across the pregnancy, with scans at the dating scan, then at 20, 24, 28, 32 and 36 weeks, plus appointments without a scan at 16 and 34 weeks. Growth is assessed at least every 28 days.

If the babies share a placenta (MCDA)

At least eleven antenatal appointments, with scans every two weeks from 16 weeks right through to 34 weeks — 16, 18, 20, 22, 24, 26, 28, 30, 32 and 34 — and then continuing to birth.

That fortnightly interval is not padding. It exists because twin-to-twin transfusion syndrome can appear and progress within a fortnight, and because catching it early is the difference between a treatable problem and a catastrophic one. If you have monochorionic twins and your scans are being spaced four weeks apart, that is worth questioning.

If the babies share a sac (MCMA)

Individualised care from a tertiary fetal medicine centre, with a planned birth by caesarean between 32+0 and 33+6 weeks.

Growth: the number to know is the discordance

In twin pregnancy the concern is not only whether each baby is growing, but whether they are growing at the same rate. The measure used is estimated fetal weight discordance:

The thresholds that change management:

• 20% discordance, or either baby below the 10th centile — monitoring increases to weekly, with umbilical artery Doppler added.
• 25% discordance together with a baby below the 10th centile — this is selective growth restriction, and it warrants referral to a tertiary fetal medicine centre.

Scan-based weight estimates carry a genuine margin of error, which is why a single discordant measurement is a trigger for closer watching rather than for immediate action. What matters is the trend across scans — which is another reason the babies must be labelled consistently.

Twin-to-twin transfusion syndrome

This is the complication that defines monochorionic twin care, and it is worth understanding properly rather than fearing vaguely.

When two babies share a placenta, they also share blood vessels running through it. If the flow across those connections becomes unbalanced, one baby steadily transfuses the other. The donor baby becomes under-filled, produces less urine and has too little fluid around it. The recipient becomes overloaded, produces too much urine and has too much fluid around it, and its heart comes under strain.

It occurs only where there is a shared placenta. Twins with separate placentas cannot develop it, which is the entire reason the two groups are watched so differently.

How it is looked for

By measuring the deepest pocket of amniotic fluid around each baby at every fortnightly scan from 16 weeks. The diagnosis is made when one baby has a deepest pocket under 2 cm while the other has a pocket over 8 cm before 20 weeks, or over 10 cm from 20 weeks onwards.

An abnormal fluid measurement that does not yet meet those criteria is not ignored: it means more frequent monitoring and review by the named obstetrician. Confirmed twin-to-twin transfusion is a tertiary fetal medicine referral, where laser treatment of the connecting vessels is the established option.

Twin anaemia polycythaemia sequence

A less well-known relative of the same problem, in which blood is transferred slowly enough that fluid volumes stay normal but one baby becomes anaemic and the other over-concentrated. It is looked for with a Doppler measurement of blood flow speed in the middle cerebral artery, done weekly from 16 weeks in pregnancies that have had treated twin-to-twin transfusion or that have selective growth restriction.

Preterm birth: the biggest single risk

Most twins arrive early, and prematurity is the principal reason twin pregnancies need close care. Two things are done about it.

Prediction. A single cervical length measurement between 16 and 20 weeks. A cervix measuring 25 mm or less identifies the pregnancies at highest risk.

Prevention. If the cervix is 25 mm or less, vaginal progesterone 200 mg once daily at bedtime, continued until 34 weeks or birth.

Equally important is what is not recommended, because a good deal of it is still offered:

• Bed rest — does not prevent preterm birth in twins, and carries its own risks including clots and deconditioning.
• Cervical stitch (cerclage) — not routine in twin pregnancy.
• Arabin pessary — not recommended routinely.
• Intramuscular progesterone injections — not recommended for this purpose.
• Oral tocolytic tablets to prevent labour — not recommended routinely.

Steroid injections to mature the babies' lungs are given when preterm birth is actually threatened, not as a routine course to every twin pregnancy at a set gestation.

What else is watched

Anaemia. Two babies draw more iron than one, and anaemia in twin pregnancy is both common and consequential — it matters more here because blood loss at delivery is greater. A full blood count at 20 to 24 weeks exists specifically to identify women who need iron early rather than late.

Pre-eclampsia. Twin pregnancy carries a higher risk. Low-dose aspirin from 12 weeks is advised where other risk factors are present alongside. Blood pressure and urine are checked at every visit, and our page on high blood pressure in pregnancy covers what is being looked for.

Gestational diabetes is also commoner in twin pregnancy, and testing follows the usual schedule — see our page on gestational diabetes.

Fetal anomaly screening is offered as in any pregnancy, with the caveat that screening for chromosomal conditions works somewhat differently in twins and should be explained specifically rather than assumed.

When are twins delivered?

Earlier than a single baby, and the date depends on chorionicity. The evidence behind each is the point at which the risk of continuing begins to exceed the risk of delivering.

Being delivered at 36 or 37 weeks is not "premature" in the sense that matters. These dates are chosen precisely because they sit at the point where waiting stops being the safer option.

Can twins be born vaginally?

Yes, and more often than the reputation suggests. For twins with separate placentas or a shared placenta with two sacs, planned vaginal birth and planned caesarean are both considered safe choices where all of the following are true:

• The pregnancy is uncomplicated and beyond 32 weeks
• There is no other obstetric reason for a caesarean
• The first twin is head-down
• There is no significant size difference between the babies

The position of the first twin is what governs the decision. The second twin's position matters much less, because after the first baby is born there is room to guide the second, and obstetricians experienced in twin birth do this routinely.

Two honest caveats belong with that. More than a third of women who plan a vaginal twin birth end up with a caesarean, which is a normal outcome rather than a failure of the plan. And a small number will need an emergency caesarean to deliver the second twin after the first has been born vaginally — uncommon, but real, and a reason twin births happen in a hospital with a theatre immediately available.

A shared sac, or triplets, means a planned caesarean. Our page on caesarean delivery covers what that involves.

Labour and delivery

A twin birth is a hospital birth with a consultant-led team, an anaesthetist and a paediatric team present or immediately available, and a theatre ready. That is not pessimism; it is what makes a planned vaginal twin birth a reasonable thing to attempt.

Practical points that are worth knowing in advance:

• Intravenous access and a group-and-save sample are arranged at the onset of labour, because the risk of heavier bleeding is higher.
• An epidural is often suggested rather than required, because if the second twin needs to be turned or delivered surgically, effective anaesthesia is already in place.
• Continuous monitoring of both babies during labour is standard.
• The third stage is actively managed — that is, with a drug to contract the uterus. Physiological management is specifically not offered in twin pregnancy, because postpartum haemorrhage is more likely with a larger placental bed and an over-distended uterus.

The possibility of needing a blood transfusion should be discussed by around 28 weeks, so that it is a conversation had calmly in advance rather than in the moment.

When a tertiary centre is needed

Some twin pregnancies should be looked after in a specialist fetal medicine centre rather than a general unit. The recognised reasons are:

• Babies sharing an amniotic sac
• Weight discordance of 25% or more with a baby below the 10th centile
• Twin-to-twin transfusion syndrome, or suspected twin anaemia polycythaemia sequence
• A fetal anomaly, or the death of one twin
• Rarer complications including TRAP sequence and conjoined twins

Knowing that this list exists is useful. If one of these applies to you and referral has not been mentioned, it is a fair thing to raise.

What happens at the clinic

A twin pregnancy is a high-risk pregnancy, and it is managed as one. Antenatal consultations, blood tests, blood pressure and urine checks, iron and other treatment, and the planning that ties the whole schedule together happen at the clinic. Scans — which in a twin pregnancy are frequent and central — are arranged by referral, since we do not run an ultrasound machine here, and specialist fetal medicine input is arranged where it is needed.

Delivery, whether vaginal or by caesarean, takes place at hospital with a full team. Nobody delivers twins anywhere else.

What a good twin pregnancy needs most is not any single intervention but continuity: the same person tracking the same two babies against the same labels, on a schedule that does not slip. That is the part worth organising early.