When Are Twins Usually Born

The number that reframes everything

NICE asks teams to explain to women with a twin pregnancy that "about 60 in 100 twin pregnancies result in spontaneous birth before 37 weeks". The NHS puts the same figure as around 6 in 10 twins born before 37 weeks. For triplets, NICE says about 75 in 100 result in spontaneous birth before 35 weeks, and the NHS says almost 8 in 10 triplets are born before 35 weeks.

ACOG, writing for a US audience, says the most common complication of multiple pregnancy is preterm birth and that "more than one half of all twins are born preterm", with triplets and more almost always born preterm.

So the single most useful thing to know about twin timing is that the date on your first scan report is not a target. It is a reference point for measuring growth.

The gestations NICE recommends for planned birth

NICE NG137 sets these out explicitly for uncomplicated pregnancies, and every one of the figures below is NICE's.

• Dichorionic diamniotic twins: offer planned birth at 37 weeks.
• Monochorionic diamniotic twins: offer planned birth at 36 weeks, after a course of antenatal corticosteroids has been considered.
• Monochorionic monoamniotic twins: offer planned birth between 32 weeks and 0 days and 33 weeks and 6 days, again after corticosteroids have been considered.
• Trichorionic triamniotic or dichorionic triamniotic triplets: offer planned birth at 35 weeks.

The NHS reproduces the same list on its antenatal care with twins page: 37 weeks for twins with a placenta each, 36 weeks for identical twins sharing a placenta, 35 weeks for an uncomplicated triplet pregnancy, and 32 to 33 weeks for monochorionic monoamniotic twins.

For a monochorionic triamniotic triplet pregnancy, or any triplet pregnancy involving a shared amnion, NICE says the timing will be decided and discussed individually rather than set by a rule.

Why the dates step down like that

NICE frames each recommendation as a balance, and states both sides. For dichorionic diamniotic twins it says planned birth from 37 weeks and 0 days "does not appear to be associated with an increased risk of serious neonatal adverse outcomes", while continuing beyond 37 weeks and 6 days "increases the risk of fetal death". For monochorionic diamniotic twins the same two statements are made at 36 weeks and 0 days and 36 weeks and 6 days. For monochorionic monoamniotic twins, planned birth between 32 weeks and 33 weeks and 6 days does not appear to increase serious neonatal adverse outcomes, while continuing beyond 33 weeks and 6 days increases the risk of fetal death. For trichorionic triamniotic or dichorionic triamniotic triplets, NICE says continuing beyond 35 weeks and 6 days increases the risk of fetal death.

NICE is also honest about the cost of the earliest date. For monochorionic monoamniotic twins it says these babies "will usually need to be admitted to the neonatal unit and have an increased risk of respiratory problems". The recommendation is not that 32 weeks is harmless. It is that the alternative is worse.

What "uncomplicated" is doing in those sentences

Every one of those gestations is prefaced by "uncomplicated". NICE says to offer an individual assessment to determine timing for any complicated twin or triplet pregnancy, for monochorionic triamniotic triplets, and for any triplet pregnancy involving a shared amnion. If TTTS, selective growth restriction, pre-eclampsia or anything else has entered the picture, the standard dates stop applying and your fetal medicine team sets a plan for your pregnancy specifically.

If you would rather not be induced or delivered on that date

NICE has a specific recommendation for this, which is worth knowing because it means declining is a recognised path rather than an argument. For women who decline planned birth at the recommended timing, NICE says to offer weekly appointments with the specialist obstetrician, and at each appointment to offer an ultrasound scan with assessment of amniotic fluid and umbilical artery Doppler for each baby, in addition to fortnightly fetal growth scans.

The NHS states the same arrangement: weekly obstetric appointments and weekly scans, plus a growth scan every two weeks. It also says plainly that "it can be dangerous for twin and triplet pregnancies to carry on after the planned birth dates".

Steroids, and what they are for

NICE's recommendations on planned birth for monochorionic diamniotic, monochorionic monoamniotic and triplet pregnancies all say that a course of antenatal corticosteroids should be considered first. It also says something that catches people out: do not use single or multiple untargeted routine courses of corticosteroids in twin or triplet pregnancy, and inform women that there is no benefit in untargeted administration. Steroids are given when birth is expected soon, not as a routine twin precaution.

Twins Trust makes the practical point that many twins and triplets spend some time in a neonatal unit or special care baby unit so they can grow, learn to feed and adjust before going home. Planning for that possibility is more useful than hoping it will not happen.

What we are not going to tell you

Recommended timings differ between countries, and it would be easy to imply more consensus than exists. ACOG's patient-facing information on multiple pregnancy does not publish a chorionicity-by-chorionicity table of delivery gestations. Its detailed timing guidance sits in a committee opinion on medically indicated late-preterm and early-term deliveries, and the full text of that document — including the delivery-timing table itself — is behind a members-and-purchasers paywall. Only the abstract is publicly readable, and the abstract does not contain the twin gestations.

So we are not quoting ACOG figures for monochorionic or dichorionic twins. The gestations on this page are NICE's, they apply to UK care, and if you are being looked after elsewhere the right question for your team is which national guideline they are working to.

The practical version

Ask two questions at your 24 to 28 week appointments: what gestation is being planned for my babies, and what is the plan if labour starts before then. NICE says both of those conversations should have happened by 28 weeks at the latest.