Twin Pregnancy Due Date Calculator
Due date and the guideline delivery window for twins or triplets — by chorionicity (dichorionic, monochorionic-diamniotic, monoamniotic), ACOG or NICE timing, from your last period, a scan, an IVF transfer or a known due date — with how far along you are, a twin-specific milestone timeline and a week-by-week calendar.
How it works
dating start = LMP + (cycle − 28) days, or scan date − gestational age, or transfer date − (14 + embryo age) days, or due date − 280 days · due date = dating start + 280 days · planned delivery = dating start + guideline weeks. ACOG: dichorionic 38w0d–38w6d (36w0d–37w6d with growth restriction), monochorionic-diamniotic 34w0d–37w6d (32w0d–34w6d with growth restriction), monoamniotic 32w0d–34w0d. NICE: dichorionic 37 weeks, monochorionic-diamniotic 36 weeks, monoamniotic 32w0d–33w6d, triplets 35 weeks.
A due date is calculated the same way for multiples as for one baby — 40 weeks (280 days) from the start of the last period, or from the equivalent date worked out from an early scan, an IVF transfer or a due date you already have. What differs is when twins and triplets are actually born. Guidelines recommend planned birth earlier because the risks of stillbirth and placental problems rise sooner, especially when the babies share a placenta. The American College of Obstetricians and Gynecologists (ACOG) suggests 38 weeks for uncomplicated dichorionic twins, 34 to 37 weeks 6 days for monochorionic-diamniotic twins and 32 to 34 weeks for monoamniotic twins; the UK’s NICE recommends 37, 36 and 32 to 33 weeks 6 days, and 35 weeks for triplets. Pick the guideline your care follows; chorionicity is on your first-trimester scan report. The calculator shows the window in dates and weeks, how far along you are, the days to the window, a milestone timeline (chorionicity scan, extra scans for shared-placenta twins, anatomy scan, glucose test, the 37-week preterm mark) and a week-by-week calendar. It is an educational estimate: your obstetric team decides the timing for your pregnancy.
Worked example
Last period 10 May 2026, dichorionic twins, ACOG timing → standard due date Sunday 14 February 2027 (10 May + 280 days); planned delivery at 38w0d to 38w6d, which is 31 January to 6 February 2027. With NICE timing the window is 37 weeks: 24 to 30 January 2027. Monochorionic-diamniotic twins under ACOG would be 34w0d to 37w6d: 3 January to 30 January 2027.
Sources: ACOG Committee Opinion 831 — Medically Indicated Late-Preterm and Early-Term Deliveries · ACOG Practice Bulletin 231 — Multifetal Gestations · NICE NG137 — Twin and triplet pregnancy
Frequently asked questions
How do I know the chorionicity?
The first-trimester scan (ideally between 11 and 14 weeks) reports it — look for “DCDA”, “MCDA” or “MCMA” on the notes. Non-identical twins are always dichorionic; identical twins can be either.
Why do ACOG and NICE give different weeks?
They weigh the same evidence slightly differently. For uncomplicated dichorionic twins ACOG suggests 38 weeks and NICE 37 weeks; for monochorionic-diamniotic twins ACOG gives a range from 34 to 37 weeks 6 days, while NICE suggests 36 weeks. Follow the guidance your care team uses.
Is an IVF twin due date different?
Only in how it is dated: choose “IVF embryo transfer”, enter the transfer date and whether it was a day-3, day-5 or day-6 embryo. The calculator converts it to the equivalent last-period date and applies the same earlier windows.
How likely is it that my twins arrive before the window?
Quite likely. About 60% of twins are born before 37 weeks, and the average twin birth is around 35 weeks. Having a hospital bag ready by about 30 weeks is sensible.
Does growth restriction change the timing?
Under ACOG it can: about 36 to 37 weeks 6 days for dichorionic twins and 32 to 34 weeks 6 days for monochorionic twins with isolated growth restriction. Switch it on under More options. Other complications change timing too, and your team decides.
Related calculators
Educational estimate only, not medical advice. Timing for twins and triplets depends on chorionicity, growth and complications — your obstetrician or maternal-fetal medicine team will confirm the dates and the delivery plan for your pregnancy.