Twin Gestational Age: One Number, Two Babies

Two babies, one gestational age. The count runs exactly as it does for a singleton — days since the first day of the last period, divided by seven. What a twin pregnancy changes is which measurement sets that date and how long the pregnancy is allowed to run.

The same count, a different measurement

Both fetuses were conceived at the same time, so there is one age. But at the dating scan there are two crown-rump lengths, and they rarely match to the millimeter. ISUOG’s practice guideline resolves it: date the pregnancy from the larger of the two crown-rump lengths before 13 weeks and 6 days, and from the head circumference of the larger twin at or beyond 14 weeks.

The rule is a compromise, and the guideline says so. Studies in pregnancies conceived by assisted reproduction — where the true age is known — found the smaller twin’s measurement correlated best with it. Dating by the larger twin is still the most common practice, and it errs toward reading the pair as slightly older, which is the safer direction for scheduling delivery.

If the pregnancy came from IVF, the measurement question is moot: ISUOG says twin pregnancies conceived that way should be dated using the age of the embryo and the date of transfer, the same rule ACOG applies to singletons. See IVF gestational age.

Size difference is not an age difference

A twin measuring a week behind the other is not a week younger. It is growing differently, and that is what gets followed. Growth discordance above 20 percent in estimated fetal weight is one of the conditions that takes a planned vaginal delivery off the table, alongside a non-cephalic presenting twin.

Forty weeks is a reference point, not a plan

The calculator above reports a 40-week due date because that is the standard anchor. Twins are rarely allowed to reach it. US delivery planning is organized by chorionicity — whether the babies share a placenta and a sac — and the windows are specific:

Twin type	Share of twin pregnancies	Planned delivery, uncomplicated
Monochorionic monoamniotic	about 8 per 100,000 pregnancies	32 0/7 to 34 0/7 weeks
Monochorionic diamniotic	about 1 in 5 twin pregnancies	34 0/7 to 37 6/7 weeks
Dichorionic diamniotic	the majority	38 0/7 to 38 6/7 weeks

Chorionicity is easiest to establish in the first trimester and gets harder later, which is the practical reason an early scan matters more in a twin pregnancy than in a singleton: the same exam that fixes the date also fixes which of those three rows you are in.

Surveillance is scheduled off that answer too. Serial growth scans start at 16 weeks for monochorionic pairs; uncomplicated dichorionic pairs get weekly antenatal testing from 36 0/7 weeks.

For the dating rules that twins share with every other pregnancy, see the gestational age calculator and gestational age by ultrasound.

This content is informational. A twin pregnancy is managed individually, and your due date and delivery plan come from your prenatal care team.