# Twin Due Date Calculator _Your 40-week medical due date, plus the planned-birth target and window for twins by chorionicity, and for triplets._ - Source: https://proba.baby/en/tools/twin-due-date-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last reviewed: 2026-09-05 ## How it works Start with the date you know best — the first day of your last period, the date you conceived, or the estimated due date your care team has already given you. Whichever you pick, the calculator resolves it to a single dating anchor and derives everything else from there, so the three modes can never disagree with each other. Then say how many babies you are carrying. For twins, one more question appears: what your 11–14 week scan report says about chorionicity — two placentas (DCDA), a shared placenta with two sacs (MCDA), or a shared placenta and a shared sac (MCMA). That is data entry off the report rather than something to guess at, and if the report is not in front of you, pick “Not sure yet” and the conservative DCDA figures are used. Chorionicity is a twins-only question; triplets and above are not classified this way here. The result always leads with your medical due date at 40+0. That is the date your care team dates the pregnancy from, and nothing on this page changes it. Underneath it sits the planned-birth target for your specific pregnancy and the window around it, taken from NICE NG137 and ACOG Practice Bulletin No. 231 — because twins and triplets are routinely planned to be born before 40 weeks, not because your due date has moved. The same table drives the multiples dates in the ProbaBaby iOS app, which requires iOS 26. Quadruplets and above get words instead of a date. No guideline sets a single planned-birth week above triplets, so the calculator says so plainly and gives you a range in prose rather than inventing a date. Every input is kept in the page URL, so you can copy the link and it reopens on exactly the same result. ## The math Every date on this page comes from one anchor: the dating LMP. In last-period mode the dating LMP is the date you entered. In conception mode it is your conception date minus 14 days, which is the same as conception plus 266 days. In due-date mode it is the due date you entered minus 280 days, so the date your care team gave you is echoed back unchanged. The medical due date is always the dating LMP plus 280 days (40+0) — Naegele's rule as described in ACOG Committee Opinion No. 700. It is calculated and shown first for every pregnancy, singleton or not, and the multiples figures are added beside it rather than substituted for it. The planned-birth target and window are a fixed number of days from that same dating LMP. Twins with two placentas (DCDA), and twins whose chorionicity has not been entered, use 238 / 259 / 272 days — 34+0 to 38+6, target 37+0. MCDA twins use 238 / 252 / 265 days — 34+0 to 37+6, target 36+0, matching NICE's planned birth from 36+0 for an uncomplicated monochorionic diamniotic pair. MCMA twins use 224 / 231 / 237 days — 32+0 to 33+6, target 33+0. Triplets use 224 / 245 / 251 days — 32+0 to 35+6, target 35+0. Worked example. For a last period of 1 January 2026, the medical due date is 8 October 2026. DCDA (or not-yet-entered) twins then target 17 September 2026, window 27 August to 30 September. MCDA twins target 10 September 2026, window 27 August to 23 September. MCMA twins target 20 August 2026, window 13 August to 26 August. Triplets target 3 September 2026, window 13 August to 9 September. The 8 October medical due date is identical in all four cases. For four or more babies the calculator returns no target and no window at all. The date fields are deliberately left empty rather than extrapolated, and the page shows a range in words instead. These are guideline planning figures, not predictions of when labour will start. A first-trimester ultrasound remains the most accurate way to date any pregnancy, and your own care team's plan takes precedence over every number here. ## FAQ **When are twins usually born?** Earlier than a single baby, and the guidelines are specific about it. NICE NG137 and ACOG Practice Bulletin No. 231 set planned-birth timing by chorionicity: from 37+0 for twins with two placentas (DCDA), from 36+0 for monochorionic diamniotic (MCDA) twins, and 32+0 to 33+6 for monochorionic monoamniotic (MCMA) twins. Those are planning targets rather than predictions — spontaneous labour can start earlier, which is why the calculator shows the surrounding window as well as the target date. **Does the twin date replace my medical due date?** No. Your 40-week medical due date is what your care team dates the pregnancy from, and it is shown first on every result here. The twin target sits alongside it and answers a different question: when a birth would typically be planned, not how far along you are. Both numbers are true at the same time, and the page never overwrites one with the other. **What is chorionicity, and why does the calculator ask?** Chorionicity describes how many placentas and amniotic sacs your twins have. Your 11–14 week scan report names it: DCDA (two placentas), MCDA (one placenta, two sacs) or MCMA (one placenta, one sac). It moves the recommended timing by up to four weeks, which is why it is asked before any date is shown. Copy across whatever the report says rather than guessing — this is data entry, not a question to answer from memory. **What if I do not know my chorionicity yet?** Pick “Not sure yet”. The calculator treats that exactly like DCDA — the latest of the twin windows — so nothing is understated while you wait for the scan report. When the report is in your hands you can change the answer and every date updates immediately. A never-answered and an explicitly-unsure answer behave identically. **Why is there no due date for quadruplets or more?** Because no guideline sets one. NICE NG137 covers twins and triplets; above that, timing is individual and usually much earlier, with higher-order multiples commonly born before 32 weeks. Rather than extrapolate a date no guideline supports, the calculator gives a range in words and leaves the timing to the team looking after you. **Can I use this if a scan already gave me a due date?** Yes, and it is the better starting point than a remembered last period. Pick “Due date you were given” and enter it: the calculator works backwards to the dating anchor, echoes your due date back unchanged, and builds the twin target and window around it. A first-trimester ultrasound is the most accurate dating method there is. **Are these the dates my twins will actually arrive?** No — they are guideline figures for when a birth would be planned, not a forecast of spontaneous labour. Twin pregnancies vary widely and many begin labour before the target date. Read the window as the realistic span and the target as the point your care team is most likely to plan around, then let their assessment override both. **How is the 40-week medical due date worked out?** Last period plus 280 days — the standard Naegele calculation described in ACOG Committee Opinion No. 700. From a conception date it is conception plus 266 days, and if you enter a due date directly that date is used as it stands. The same 40-week date is calculated for twins and triplets as for a single baby; carrying more than one does not change how the pregnancy is dated. ## Sources - NICE NG137 — Twin and triplet pregnancy — National Institute for Health and Care Excellence, 2019 (planned birth timing by chorionicity): https://www.nice.org.uk/guidance/ng137 - ACOG Practice Bulletin No. 231 — Multifetal Gestations — American College of Obstetricians and Gynecologists, Obstet Gynecol 2021;137:e145–62: https://www.acog.org/clinical/clinical-guidance/practice-bulletin/articles/2021/06/multifetal-gestations-twin-triplet-and-higher-order-multifetal-pregnancies - ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists, 2017 (the 40-week medical due date): https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date --- 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.