# When Are Twins Born? Twin Delivery Timing by Chorionicity (34–38 Weeks)

_Planned birth is offered at 37, 36 or 32 to 33 weeks depending on what your scan report says — and around 6 in 10 twins arrive before 37 weeks anyway._

- Source: https://proba.baby/en/blog/when-are-twins-born
- Published: 2026-09-05
- Author: ProbaBaby Team
- Category: Twins & Multiples
- Tags: twins, multiples, labor, third-trimester
- Language: English

> Planned twin birth falls between 34 and 38 weeks in UK and US guidance, and the exact week depends on chorionicity: UK guidance offers planned birth at 37 weeks for twins with a placenta each, 36 weeks when they share a placenta, and 32 to 33 weeks when they share a sac. Around 6 in 10 twins arrive before 37 weeks regardless.


> **Medical disclaimer.** This article explains how twin birth timing is planned in published national guidance and what the numbers say about when twins actually arrive. It is not medical advice. The week that applies to you is the one your own obstetric team sets, from your scans, your chorionicity and your own pregnancy — and national guidelines differ. If you have bleeding, pain, contractions, waters breaking or reduced movements at any gestation, contact your maternity unit straight away rather than waiting for your next appointment.

**When are twins born?** For most twin pregnancies, somewhere between 34 and 38 weeks — every planned-birth window in UK and US guidance for twins with two sacs falls inside that span. There is no single answer because there is no single kind of twin pregnancy. UK guidance offers planned birth at **37 weeks** for twins with a placenta each, **36 weeks** for identical twins sharing a placenta, and **32 to 33 weeks** for twins sharing a placenta *and* a sac. And the plan is only half the story: the NHS states that around **6 in 10 twins are born before 37 weeks**.

So a twin due date is not really a date. It is a target with a window around it, and knowing how wide that window is turns out to be the most practical thing you can carry into your third trimester.

## The three planned-birth weeks

Once your first-trimester scan settles chorionicity — whether your babies share a placenta, and whether they share an amniotic sac — the default planned birth week follows from it, complications aside. This is how NICE NG137 and the NHS lay it out:

| Type of twin pregnancy | Planned birth (NICE NG137 / NHS) |
|---|---|
| Di/di — DCDA, a placenta each | 37 weeks (37+0 to 37+6) |
| Mo/di — MCDA, one placenta, two sacs | 36 weeks (36+0 to 36+6) |
| Mo/mo — MCMA, one placenta, one sac | 32+0 to 33+6 |
| Uncomplicated triplets | 35 weeks |

ISUOG's 2025 twin ultrasound guideline lands in the same place from a different direction: "the optimal gestational age for delivery of uncomplicated dichorionic twins is considered to be between 37 + 0 and 37 + 6 weeks, and that for uncomplicated monochorionic twins between 36 + 0 and 36 + 6 weeks."

Read the table again and notice the spread. The earliest twin plan starts at 32+0 and the latest runs to 37+6 — almost six weeks between them — and the thing that decides which row you are on is a single line on a scan report.

## Why chorionicity moves the date by weeks

Twins who have a placenta each are, in circulation terms, two separate pregnancies sharing a uterus. Twins who share one placenta also share blood vessels running through it, and that connection is what the extra monitoring is watching — it is why the same UK unit guideline that scans dichorionic twins for growth every 4 weeks from 24 weeks scans monochorionic twins **every 2 weeks from 16+0**.

ISUOG gives the reason for the earlier week plainly: past 36+0 to 36+6 for uncomplicated monochorionic twins, "prolongation of pregnancy beyond this stage may increase the risk of perinatal mortality". Sharing a single sac as well adds cord entanglement, which ISUOG calls almost universal in MCMA pregnancies — which is why mo/mo twins sit at 32+0 to 33+6, the earliest planned birth of any uncomplicated twin pregnancy, and why ISUOG says these pregnancies should be referred to a tertiary centre with expertise in managing them.

None of this is a verdict on how your pregnancy will go. It is why "when are twins born" has three answers instead of one, and why the answer arrives at your dating scan rather than at 36 weeks. If you want the full version of what the labels mean and how sonographers tell them apart, we wrote that up separately in [di/di, mo/di and mo/mo twins explained](/blog/chorionicity-di-di-mo-di-mo-mo-twins).

## If your doctor says 38 weeks, that is not a contradiction

US practice sits later at the di/di end. The StatPearls chapter on multiple birth delivery, which cites ACOG Practice Bulletin No. 231, gives **38 0/7 to 38 6/7 weeks** for uncomplicated dichorionic-diamniotic twins, **34 0/7 to 37 6/7** for monochorionic-diamniotic, and **32 0/7 to 34 0/7** for monochorionic-monoamniotic.

Put the two side by side and the mo/di row is where the framings differ most: the US window opens at 34+0 and runs four weeks wide, while UK guidance names a single week at 36+0. If your obstetrician quotes a week this article does not, you are almost certainly looking at two guideline traditions rather than an error. The useful question at your next appointment is simply: *which guideline does this unit follow, and what is my planned week?*

## Most twins do not wait for the plan

This is the part that reframes everything above. The NHS says around 6 in 10 twins are born before 37 weeks, and almost 8 in 10 triplets before 35 weeks. NICE's full multiple pregnancy guideline (CG129, 2011) states that "more than 50% of twins and almost all triplets are born before 37 weeks of gestation", and that about 15–20% of neonatal unit admissions are associated with preterm twins and triplets.

It is also worth knowing *how* twins arrive early, because "preterm" sounds like one thing and is really three. In an Italian area-based prospective cohort of 346 twin deliveries in the late preterm window (34+0 to 36+6), 85 (24.6%) followed spontaneous preterm labour, 66 (19.1%) followed waters breaking early, and 195 (56.3%) were medically indicated. Some twins start labour on their own; more are delivered on a clinical decision, often earlier than the standard plan because something in the monitoring changed.

Either way, arriving before 37 weeks is the typical twin story, not the emergency version of it. Planning for it is not pessimism.

## Plan for a window, not a date

The practical consequence is a calendar shift of several weeks. Everything you would normally do "near the end" moves earlier in a twin pregnancy: the hospital bag packed and by the door, car seats fitted and tested, the childcare plan for an older child agreed with a named person, the hospital route and phone numbers written where a panicking partner can find them. Our [hospital bag checklist guide](/blog/hospital-bag-checklist-guide) is worth working through weeks earlier than a singleton timeline suggests.

It is also worth knowing what happens if you decline the offer of planned birth. The NHS describes the alternative plainly: weekly appointments with an obstetrician, weekly ultrasound scans, and a fetal growth scan every 2 weeks. Declining is a real option with a real monitoring plan attached, not a shrug.

## Your 40-week due date does not go away

One thing the twin target does not do is replace your estimated due date. The 40-week EDD is the anchor every week number in your pregnancy is measured from — your scan reports, your growth charts, your test windows and the "you are 31+4 today" line all count from it, and it stays in your notes to the end.

What the twin target adds is a second, earlier date to prepare around. Any app or calculator that quietly swaps one for the other has broken the arithmetic every other number in your notes depends on; the longer version of that argument is in [your twin due date, explained](/blog/twin-due-date-explained). If you want both dates for your own chorionicity, our [twin due date calculator](/tools/twin-due-date-calculator) gives the medical EDD and the twin target and window side by side, with the guideline it uses named on screen.

## How ProbaBaby helps

Declare twins in your ProbaBaby pregnancy profile and the timeline shows a twin target of 37+0 with a 34+0 to 38+6 window, while your medical 40-week due date is never overwritten. Adding chorionicity from your scan report refines that target to 36+0 (window 34+0 to 37+6) for MCDA twins and 33+0 (window 32+0 to 33+6) for MCMA; triplets get 35+0 with a 32+0 to 35+6 window, and quadruplets or more show a text range rather than a computed date. The question is phrased as reading a line off your 11–14 week scan report, never as something to answer from memory, and "not sure yet" is a first-class answer you can change later. Because a twin arriving at 35 weeks would otherwise blow past deadlines set for 40, birth-prep checklist deadlines and the delivery milestone anchor on the twin target — with the medical due date carried along in the exported calendar event's notes — and the doctor-visit summary and its PDF print the standard and adjusted delivery windows as separate rows, with your chorionicity carried on the multiples row once you have entered it, so your team can see exactly which dates you have been working to. The [twins page](/twins) covers how the rest of the twin timeline fits together.

## FAQ

**When are twins usually born?**

Planned twin birth falls between 34 and 38 weeks across UK and US guidance, and most twins arrive before 37 weeks. UK guidance offers planned birth at 37 weeks for twins with a placenta each, 36 weeks for identical twins sharing a placenta and 32 to 33 weeks for twins sharing a sac, but the NHS also says around 6 in 10 twins are born before 37 weeks whatever the plan says.

**Why does chorionicity change the delivery date?**

Twins who share a placenta share blood vessels through it, and ISUOG notes that continuing past the recommended week may increase the risk of perinatal mortality, so guidance brings the planned birth week forward. That is why di/di twins are offered 37 weeks, mo/di twins 36 weeks and mo/mo twins 32+0 to 33+6 — planned birth weeks as much as five weeks apart, decided by one line on your scan report.

**Is 37 weeks full term for twins?**

37 weeks is not a separate twin definition of term — it is the week UK guidance offers planned birth for uncomplicated twins with a placenta each. Your notes still count to a 40-week due date, but for di/di twins that 37-week offer is the week that behaves like the finish line.

**Why does my US doctor say 38 weeks when UK guidance says 37?**

Because they are two guideline traditions, not a contradiction. The StatPearls multiple-birth chapter gives 38 0/7 to 38 6/7 weeks for uncomplicated dichorionic-diamniotic twins, while NICE NG137 and the NHS offer 37 weeks. Ask your team which guideline your unit follows.

**How many twins are born prematurely?**

The NHS states that around 6 in 10 twins are born before 37 weeks, and almost 8 in 10 triplets before 35 weeks. NICE's full multiple pregnancy guideline (CG129) puts it as more than 50% of twins and almost all triplets born before 37 weeks.

**Do twins mostly arrive on their own or by planned birth?**

Both routes are common. In an Italian prospective cohort of 346 twin deliveries between 34+0 and 36+6 weeks, 85 (24.6%) followed spontaneous preterm labour, 66 (19.1%) followed waters breaking early and 195 (56.3%) were medically indicated. Arriving before 37 weeks is the typical case, not the emergency case.

**Does a twin target date replace my 40-week due date?**

No. The 40-week estimated due date is what every week number, scan report and growth chart is measured from, and it stays in your notes. A twin target is an extra date to prepare around, not a replacement.

## Sources

- NHS — Antenatal care with twins — Planned birth at 37 weeks (a placenta each), 36 weeks (sharing a placenta), 32-33 weeks (MCMA), 35 weeks (uncomplicated triplets); around 6 in 10 twins born before 37 weeks and almost 8 in 10 triplets before 35 weeks; weekly obstetrician appointments, weekly scans and a growth scan every 2 weeks if planned birth is declined: https://www.nhs.uk/pregnancy/your-pregnancy-care/antenatal-care-with-twins/
- NICE Multiple Pregnancy full guideline (CG129, 2011) — Preterm birth (NCBI Bookshelf NBK83112) — More than 50% of twins and almost all triplets are born before 37 weeks of gestation; about 15-20% of neonatal unit admissions are associated with preterm twins and triplets; extreme prematurity is also more frequent in twin and triplet pregnancies: https://www.ncbi.nlm.nih.gov/books/NBK83112/
- NHS Greater Glasgow & Clyde — Twin Pregnancy Ultrasound Guideline (330) — NICE NG137-aligned UK unit guideline — delivery 37+0 to 37+6 for uncomplicated DCDA, 36+0 to 36+6 for uncomplicated MCDA, 32+0 to 33+6 for MCMA; growth scans every 4 weeks from 24 weeks (DCDA) and every 2 weeks from 16+0 (monochorionic): https://www.rightdecisions.scot.nhs.uk/ggc-clinical-guidelines/maternity/ultrasound/twin-pregnancy-ultrasound-guideline-330/
- StatPearls — Multiple Birth Delivery (NCBI Bookshelf NBK599524) — US-side planned delivery windows: uncomplicated di-di 38 0/7-38 6/7, mo-di 34 0/7-37 6/7, mo-mo 32 0/7-34 0/7; delivery timing for triplets and higher-order pregnancies is individualised: https://www.ncbi.nlm.nih.gov/books/NBK599524/
- ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy — Khalil A, Sotiriadis A, Baschat A, et al. Ultrasound Obstet Gynecol 2025;65(2):253-276 — optimal delivery 37+0 to 37+6 for uncomplicated dichorionic and 36+0 to 36+6 for uncomplicated monochorionic twins, because 'prolongation of pregnancy beyond this stage may increase the risk of perinatal mortality'; scans every 4 weeks (dichorionic) and every 2 weeks after 16 weeks (monochorionic); cord entanglement almost universal in MCMA, which should be referred to a tertiary centre with expertise in their management: https://pmc.ncbi.nlm.nih.gov/articles/PMC11788470/
- Monari F, Chiossi G, Ballarini M, et al. Perinatal outcomes in twin late preterm pregnancies. Ital J Pediatr 2022;48:101 — Italian area-based prospective cohort (Emilia Romagna, 21 hospitals, 2013-2015), 346 twin deliveries at 34+0-36+6 — 85 (24.6%) after spontaneous preterm labour, 66 (19.1%) after pPROM, 195 (56.3%) medically indicated (117 evidence-based, 78 non-evidence-based): https://pmc.ncbi.nlm.nih.gov/articles/PMC9204959/

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