When Are Twins Usually Born? The Weeks, Explained
Most twins are born earlier than singletons. ACOG says “more than one half of all twins are born preterm,” which means before 37 weeks. When the pregnancy goes smoothly and birth is planned, the usual window depends on the placentas. Twins with two placentas (di-di) are commonly delivered at around 37 to 38 weeks, twins sharing a placenta (mono-di) at around 34 to 37 weeks, and twins sharing one sac (mono-mono) at around 32 to 34 weeks. Those are population ranges from guidelines. Your own date is set by your OB or maternal-fetal medicine team, and it can move.
The headline numbers
Two sources put the same fact two ways:
- ACOG’s patient page on multiple pregnancy says preterm birth is the most common complication of carrying twins, and that more than half of twins are born before 37 weeks. Triplets “are almost always born preterm.”
- The UK’s NICE guideline on twin and triplet pregnancy tells clinicians to explain that “about 60 in 100 twin pregnancies result in spontaneous birth before 37 weeks”, meaning labour that starts on its own. For triplets the figure is about 75 in 100 before 35 weeks.
Read those as facts about large groups of pregnancies. They do not predict when your twins will come.
The planned-birth window depends on chorionicity
When nothing has gone wrong, guidelines still recommend not carrying twins as long as a singleton. The timing follows chorionicity: whether the twins have their own placentas and sacs or share them. (If those terms are new, what sharing a placenta means covers the difference.)
| Type | US range (ACOG, via a hospital guideline) | UK (NICE) |
|---|---|---|
| Di-di: two placentas, two sacs | 37+0 to 38+6 weeks | Planned birth offered at 37 weeks |
| Mono-di: one placenta, two sacs | 34+0 to 37+6 weeks | Planned birth offered at 36 weeks |
| Mono-mono: one placenta, one sac | Individual plan | 32+0 to 33+6 weeks |
The US column comes from the Harvard-affiliated CRICO guideline on managing twins, which cites ACOG Practice Bulletin 231. It says uncomplicated di-di twins “should usually be delivered between 37 0/7 and 38 6/7 weeks” and uncomplicated mono-di twins “may be delivered between 34 0/7 and 37 6/7 weeks.” The mono-mono row in the UK column is from NICE. In practice mono-mono pregnancies get an individual plan from a specialist team, often with a period of hospital monitoring beforehand, depending on the centre.
These windows apply to uncomplicated pregnancies. NICE says a complicated twin pregnancy needs “an individual assessment” to set the date. That can mean earlier, sometimes much earlier.
Why the window is earlier for twins
NICE gives the reason for each type in the same form: planned birth from the start of the window “does not appear to be associated with an increased risk of serious neonatal adverse outcomes,” while carrying on past the end of it “increases the risk of fetal death.” For di-di twins that cut-off is after 37+6 weeks. For mono-di twins it is after 36+6, and for mono-mono twins after 33+6.
So the window is a trade-off. Earlier birth carries the risks of prematurity. Later birth carries the risks of staying pregnant. The point where those balance comes sooner for twins, and sooner again when they share a placenta.
NICE also says that for mono-di and mono-mono twins, planned birth is offered “after a course of antenatal corticosteroids has been considered”. These are steroid shots given before an early birth to help the babies’ lungs. If your date is before 37 weeks, ask whether that applies to you.
If you decline a planned birth at the recommended time, NICE says you should be offered weekly specialist appointments with scans for each baby. Declining is allowed. It just means more monitoring.
What “born early” means once they arrive
ACOG says babies born before 37 weeks may have more short-term and long-term health problems, “including problems with breathing, eating, and staying warm.” NICE notes that both spontaneous and planned preterm birth raise the chance of a stay in the neonatal unit. Mono-mono twins delivered at 32 to 34 weeks “will usually need to be admitted.”
That is why twin families plan around the NICU before birth. The practical side, including what happens when one twin comes home first, is in one twin home, one in the NICU.
It also changes how you read milestones later. Preterm twins are usually tracked by adjusted age (counted from the due date) for growth, sleep and development, so two twins born at 35 weeks are about five weeks “younger” than their birthday says.
What to do with this before 30 weeks
You can’t choose the date, but you can be ready early:
- Ask your chorionicity at the first scan if nobody has told you. The rest of the timeline hangs on it.
- Ask the timing question by the third trimester, in plain words: “What week are we aiming for, and what would change it?” The twin pregnancy week-by-week guide has the full set of questions for each stage.
- Be ready by 34 weeks. Car seats installed, two sleep spaces set up and the bag packed, using the third-trimester twin checklist.
- Ask what level of NICU your delivery hospital has. If your twins might need a higher level, the answer can change where you deliver.
FAQ: when twins are born
What is full term for twins?
Preterm means before 37 weeks for twins too, so the definition does not change. But because planned birth for uncomplicated twins is commonly offered at 36 to 38 weeks depending on chorionicity, a twin birth in that range is the expected outcome, not an early surprise.
Can twins go to 40 weeks?
Guidelines advise against it. NICE says carrying an uncomplicated di-di pregnancy past 37+6 weeks increases the risk of fetal death, and the US range for di-di twins ends at 38+6 weeks. Some twins arrive later than that, but your OB will offer a planned birth before then.
Are identical twins born earlier?
Yes, when they share a placenta. The NHS notes that twins who share a placenta are identical, and those pregnancies have earlier planned windows. Identical twins with two placentas follow the same di-di timing as fraternal twins. It is the placenta that sets the window, not whether the twins are identical.
