# Your Twin Due Date Is Really Week 37: How Twin Due Dates Work _The 40-week date is still yours. It is just not the week your babies are being planned for._ - Source: https://proba.baby/en/blog/twin-due-date-explained - Published: 2026-09-05 - Author: ProbaBaby Team - Category: Twins & Multiples - Tags: twins, multiples, calculators, first-trimester - Language: English > A twin due date is calculated the same way a singleton's is — last period plus 280 days. What changes is the week your team plans delivery for: 37 weeks for twins with a placenta each, 36 for twins sharing one, earlier again for mo/mo twins. Here is where both numbers come from. > **Medical disclaimer.** This article explains how twin pregnancies are dated and when twins are usually delivered. It is general information, not medical advice, and no app or calculator sets a delivery plan. Your chorionicity, your scans and your own history decide your timing, so take any question about your dates to your obstetric team — and contact them straight away if you have bleeding, contractions, reduced movement, or any symptom that worries you. Your twin due date is worked out exactly the way a singleton's is: last menstrual period plus 280 days, or whatever date your first-trimester scan sets. That 40-week date is real, it goes in your notes, and every week number in your pregnancy is counted from it. It is also not the week your babies are being planned for. UK guidance offers planned birth at **37 weeks** for uncomplicated twins with a placenta each, and earlier for twins that share one — and around 6 in 10 twins arrive before 37 weeks without waiting to be offered anything. So a twin pregnancy carries two numbers: the medical due date the whole calendar is measured from, and a target week you are actually preparing for. Here is where each comes from — and why you need to be holding both. ## The formula does not change for twins Naegele's rule — first day of your last period plus 280 days — is what every general due date calculator runs, twins or not. [Our due date calculator](/tools/due-date-calculator) uses it in last-period mode, and [how due date calculators work](/blog/how-due-date-calculators-work) walks through the conception and IVF variants. [The twin version](/tools/twin-due-date-calculator) runs the identical 280-day arithmetic and then puts the planned-birth target and window for your chorionicity underneath the result, instead of in place of it. Carrying two babies changes none of that arithmetic. What it changes is the appointment that comes next. ## How twins are dated at the scan For twins, the first-trimester scan does two jobs at once, and both are time-limited. The ISUOG practice guidelines, updated in 2025, say that in twin pregnancies conceived spontaneously **the larger of the two crown-rump lengths should be used** to estimate gestational age — not an average of the two, and not the smaller baby. At or beyond 14 weeks, dating switches to the head circumference of the larger twin. Twins conceived through IVF are dated from the age of the embryo and the date of transfer, exactly as singletons are. The same appointment settles chorionicity: whether your babies have a placenta each or share one, and whether they share an amniotic sac — [what di/di, mo/di and mo/mo actually change](/blog/chorionicity-di-di-mo-di-mo-mo-twins) goes through that in full. ISUOG says to determine it before 13+6 weeks using as many features as possible — the lambda or T sign where the membrane meets the placenta, the membrane thickness, the number of placental masses — and to keep the image in your records. The NHS says the same thing in plainer words: the 11 to 14 week scan is when they find out what type of placenta and membranes your twins have, and check your dates. That makes the 11 to 14 week scan the appointment to protect. And if it cannot be settled, NICE is unambiguous about what happens next: manage the pregnancy as a monochorionic pregnancy until proven otherwise. The cautious answer wins, because monochorionic twins are watched more closely and delivered earlier. ## The week that actually gets planned Once chorionicity is known, your planned birth week is known too. NICE NG137 sets it out directly: | Twin or triplet type | NICE NG137 planned birth | |---|---| | Dichorionic diamniotic (di/di) — a placenta each | 37 weeks | | Monochorionic diamniotic (mo/di) — one placenta, two sacs | 36 weeks | | Monochorionic monoamniotic (mo/mo) — one placenta, one sac | 32+0 to 33+6 weeks | | Trichorionic or dichorionic triamniotic triplets | 35 weeks | US practice sits a little later at the di/di end. The StatPearls chapter on multiple birth delivery gives 38 0/7 to 38 6/7 weeks for uncomplicated dichorionic-diamniotic twins, 34 0/7 to 37 6/7 weeks for monochorionic-diamniotic, and 32 0/7 to 34 0/7 weeks for monochorionic-monoamniotic. If your obstetrician says 38 weeks and this article says 37, neither is wrong — you are looking at two guideline traditions. Ask which one your unit follows. That is the number that applies to you, and it is worth writing down, because it is the date every other plan hangs off. ## Why the 40-week date never goes away It is tempting to think of the twin target as a corrected due date that replaces the old one. It is not, and treating it that way breaks things quietly. Gestational age is counted from the 40-week anchor. "Delivered at 37 weeks" only has meaning because 40 weeks is the reference point. Your anomaly scan window, your growth scan cadence, the week steroids would be considered, the percentile your babies' measurements are read against — all of it is keyed to a gestational age derived from that date. Shift the anchor and every one of those references silently moves with it. There is also the plain human version: your notes, your midwife, your maternity leave paperwork and your family all speak the 40-week date. An app that quietly rewrites it hands you a number nobody else in the room recognises. The right shape is both. Keep the medical estimated due date exactly where it is, and put the twin target beside it. This is also where most pregnancy apps stop, by their own documentation. Checked on 5 September 2026: What to Expect advertises a twin mode whose documented form is twin baby-size comparisons, with the tracker still keyed to a single due date; Pregnancy+ documents twins as a setting under "Baby's sex" plus adjusted 3D scan artwork for a handful of early weeks; Flo lets you state the number of babies, and its help centre returns no results for chorionicity. Those are the vendors' own help pages and store listings rather than a hands-on test — but none of them describes a second date, which is why so many twin parents end up carrying the target week in their head. ## Plan for a window, not a day Even the target is a planning date rather than a prediction. NICE tells clinicians to explain that about 60 in 100 twin pregnancies result in spontaneous birth before 37 weeks, and the NHS puts it as around 6 in 10 twins born before 37 weeks. A meaningful share of twin parents never reach the planned date at all. [When twins are actually born](/blog/when-are-twins-born) goes through the distribution behind that number. It has one very practical consequence here: every deadline you set should sit against the earlier number, not the 40-week one. A bag packed for week 38 is a bag packed too late — [our hospital bag guide](/blog/hospital-bag-checklist-guide) covers what to bring, and for twins the honest advice is to have it standing by the door several weeks before the target. The same goes for the car seats, the birth preferences conversation, and the childcare-for-the-older-one plan. It also gives you something concrete to raise at your next appointment. Bring both dates and ask directly: which week are we planning for, and what would move it earlier? Writing the answer down is the difference between leaving with a date and leaving with a vague sense that it will be "sometime around 37 weeks". ## How ProbaBaby helps ProbaBaby's Due Date Calculator has a 1 / Twins / Triplets selector, routed through the same gestation table the rest of the app uses and citing NICE NG137 and ACOG Practice Bulletin No. 231 on screen. Chorionicity is asked separately, in the pregnancy profile, and only of twins — triplets and above never see the question. Record what your 11 to 14 week scan report says — DCDA, MCDA, MCMA or "not sure yet" — and the target moves with it: 37+0 for di/di twins and for twins whose chorionicity is not recorded yet, 36+0 for mo/di, 33+0 for mo/mo, each with its own window. Triplets get 35+0. Four or more babies get a range in words rather than a computed date, because no guideline sets a single planned week above triplets. The medical 40-week due date is never overwritten. The twin target is shown alongside it, and a twin profile gets a one-time explainer on the Home card saying so in as many words — suppressed for shared-placenta profiles, because its "week 37-38" wording would be wrong for them. Since twins commonly arrive before the target, birth-prep checklist deadlines and the delivery milestone anchor on the target date rather than the 40-week one, and the medical due date still rides along in the exported calendar event. The doctor-visit summary and its PDF print both rows — standard and adjusted, whenever your recorded dates support an adjusted one — so the appointment conversation starts from the same two numbers this article is about. You can read more on the [twins hub](/twins). ## FAQ **Is a twin due date calculated differently from a singleton due date?** No. The estimated due date is still the last menstrual period plus 280 days, or the date your first-trimester scan sets. What changes for twins is the week your team plans delivery for, which is earlier than 40 weeks. **At what week are twins usually delivered?** NICE NG137 offers planned birth at 37 weeks for uncomplicated dichorionic diamniotic twins, 36 weeks for monochorionic diamniotic twins, and between 32+0 and 33+6 weeks for monochorionic monoamniotic twins. US practice runs slightly later at the di/di end, with 38 0/7 to 38 6/7 weeks quoted for uncomplicated di/di twins. **How is a twin pregnancy dated at the scan?** ISUOG's guidelines say that in twin pregnancies conceived spontaneously, the larger of the two crown-rump lengths should be used to estimate gestational age. At or beyond 14 weeks the head circumference of the larger twin is used instead, and twins conceived by IVF are dated from the age of the embryo and the date of transfer. **Does chorionicity change my due date?** It does not change the estimated due date, which is fixed by dating. It changes the planned birth week: sharing a placenta moves the offer earlier, to 36 weeks for mo/di twins and to 32+0 to 33+6 weeks for mo/mo twins. **What if nobody has told me my chorionicity?** Ask for the 11 to 14 week scan report — that is the appointment where it is established. NICE is explicit that if chorionicity cannot be determined, the pregnancy is managed as monochorionic until proven otherwise. **Can I use an ordinary due date calculator if I am expecting twins?** Yes, for the 40-week estimated due date, because the formula is identical. What a 40-week calculator cannot give you is the planned-birth week for your chorionicity, so you need that number alongside it — our twin due date calculator shows both on one result. **Do twin pregnancies ever go past 40 weeks?** Very rarely. NICE NG137 says about 60 in 100 twin pregnancies result in spontaneous birth before 37 weeks, and the NHS puts the same thing as around 6 in 10 twins born before 37 weeks. NICE's rationale for offering birth at 37 weeks is recommendation 1.9.4: continuing beyond 37+6 weeks increases the risk of fetal death in dichorionic twins. ## Sources - NICE NG137 — Twin and triplet pregnancy (full guideline) — Published 2019-09-04, updated 2024. Verified 2026-09-05. Rec 1.9.9 planned birth at 37 weeks (DCDA); 1.9.10 36 weeks MCDA, 32+0–33+6 MCMA, 35 weeks trichorionic/dichorionic triamniotic triplets (1.9.11 monochorionic triplets assessed individually); 1.9.1 about 60 in 100 twin pregnancies result in spontaneous birth before 37 weeks; 1.9.4 continuing beyond 37+6 weeks increases the risk of fetal death in DCDA twins; 1.1.7 manage as monochorionic if chorionicity cannot be determined: https://www.ncbi.nlm.nih.gov/books/NBK546070/ - NHS — Antenatal care with twins — Last reviewed 14 October 2022. Verified 2026-09-05. The 11–14 week scan is "the best time to find out what type of placenta and membranes your twins have (chorionicity) and check your dates"; around 6 in 10 twins are born before 37 weeks; plain-language birth timings 37 weeks (a placenta each), 36 (identical sharing a placenta), 32–33 (MCMA), 35 (triplets): https://www.nhs.uk/pregnancy/your-pregnancy-care/antenatal-care-with-twins/ - Khalil A, Sotiriadis A, Baschat A, et al. ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy. Ultrasound Obstet Gynecol 2025;65(2):253–276 — Date spontaneous twins on the larger crown-rump length; head circumference of the larger twin at or beyond 14 weeks; IVF twins dated from embryo age and transfer date; chorionicity determined before 13+6 weeks from the lambda or T sign, membrane thickness and number of placental masses, with the image kept in the records: https://pmc.ncbi.nlm.nih.gov/articles/PMC11788470/ - StatPearls — Multiple Birth Delivery (NCBI Bookshelf NBK599524) — Last updated 10 January 2024, verified 2026-09-05. US-side delivery windows attributed to ACOG/SMFM: 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. Used here because ACOG Practice Bulletin No. 231 itself is paywalled: https://www.ncbi.nlm.nih.gov/books/NBK599524/ --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.