Twin due date calculator

Your due date has not moved. Your birth date probably has.

The single most common mistake in twin due date tools is to subtract three or four weeks from 40 and call the result a due date. It is not a due date. It is a planned birth date, and confusing the two produces a number that contradicts every scan report you are handed for the next six months.

Gestational age is gestational age. Your babies are 20 weeks when they are 20 weeks, and your estimated date of delivery is still 40 weeks from day zero. What changes with twins is the point at which the balance tips: the gestation beyond which continuing the pregnancy carries more risk than ending it. That is the number your team is actually planning around, and it depends almost entirely on how the placentas are arranged.

Chorionicity is the word that matters, not identical

People ask whether twins are identical. Clinicians ask about chorionicity, and the two questions are not the same. Chorionicity describes how many placentas there are and how many amniotic sacs, and NICE says explicitly that the risks in a twin pregnancy are determined by chorionicity and not by zygosity.

Dichorionic diamniotic twins have a placenta each and a sac each. Monochorionic diamniotic twins share a placenta but have separate sacs, which introduces the possibility of an unbalanced blood supply between them. Monochorionic monoamniotic twins share both, which adds the risk of cord entanglement. Each step up that ladder brings the recommended birth date forward.

Chorionicity is easiest to establish before 14 weeks, from the number of placental masses, the thickness of the membrane and the lambda or T-sign. If it cannot be determined, NICE is clear: manage the pregnancy as monochorionic until proven otherwise. That is deliberately cautious, and this tool follows it.

What NICE actually recommends

For an uncomplicated dichorionic diamniotic twin pregnancy, planned birth at 37 weeks. For an uncomplicated monochorionic diamniotic pregnancy, 36 weeks, after a course of antenatal corticosteroids has been considered. For monochorionic monoamniotic twins, between 32+0 and 33+6 weeks, again after considering steroids — and NICE notes that these babies will usually need admission to a neonatal unit. For trichorionic or dichorionic triamniotic triplets, 35 weeks.

The steroid clause is not a footnote. A course of antenatal corticosteroids given before a planned preterm or early-term birth reduces a baby's risk of breathing problems, and it is a conversation your team should start well before the date rather than on the day.

Most twin pregnancies never reach the planned date

This is the part that reframes everything. NICE tells clinicians to explain that about 60 in 100 twin pregnancies result in spontaneous birth before 37 weeks, and about 75 in 100 triplet pregnancies before 35 weeks. The planned date is a ceiling, not a prediction.

Broken down by type, the numbers are starker still. The ISUOG guideline reports birth before 37 weeks in 100% of monochorionic monoamniotic pregnancies, 88.5% of monochorionic diamniotic and 48.6% of dichorionic diamniotic. Before 32 weeks the figures are 26.8%, 14.2% and 7.4%. Knowing that most twins arrive before the date on the plan is not a reason to panic; it is a reason to have the hospital bag ready earlier than you would for one baby, and to know where the neonatal unit is.

Guidelines disagree, and pretending otherwise helps nobody

ACOG delivers uncomplicated dichorionic diamniotic twins at 38 0/7 to 38 6/7 weeks. NICE offers 37. ISUOG puts the optimal window at 37+0 to 37+6. That is a genuine week of disagreement between major bodies, resting on how each weighs a small rise in stillbirth risk against a small rise in neonatal problems.

If you are being cared for in the UK you will be offered the NICE date. If you are in the US you may be offered a week later. Neither is wrong, and it is a perfectly reasonable thing to ask your obstetrician about.

Why twin pregnancies get so many more scans

A dichorionic diamniotic twin pregnancy is offered at least eight antenatal appointments, with scans at 20, 24, 28, 32 and 36 weeks. A monochorionic diamniotic pregnancy is offered at least eleven, with scans every fortnight from 16 weeks to 34. The extra scans in monochorionic pregnancies exist to catch twin-to-twin transfusion syndrome and growth discordance early, when something can still be done about them.

The calculator lays that schedule out as dates against your own pregnancy, which is more useful than a list of week numbers when you are trying to book time off work.

When the dates change

All of the above assumes an uncomplicated pregnancy. NICE is explicit that timing is assessed individually for any complicated twin or triplet pregnancy, including twin-to-twin transfusion syndrome, significant growth discordance or a single fetal death. Monochorionic triamniotic triplets and any pregnancy with a shared amnion are decided case by case, and this tool deliberately does not guess at them.

And the rule that overrides every date on this page: reduced or changed movements in either baby is a same-day call to your maternity unit, at any gestation.