IVF Gestational Age Calculator (Day 3 or Day 5)

An IVF pregnancy is the one case where the date of conception is known to the day. ACOG’s rule follows from that: use the age of the embryo and the date of transfer, and ignore everything else. Enter both below.

The rule, and the two numbers it produces

Committee Opinion 700 gives the calculation directly. For a day-5 embryo, the estimated due date is 261 days from the replacement date. For a day-3 embryo, it is 263 days.

Both numbers come from the same place. A pregnancy runs 266 days from fertilization; subtract the days the embryo already spent developing in the lab, and what remains is counted from the transfer:

• Day 5 blastocyst: 266 − 5 = 261 days
• Day 3 cleavage-stage embryo: 266 − 3 = 263 days

Gestational age works backward from the same fact. Since the count starts two weeks before fertilization, on the day of a blastocyst transfer the pregnancy is already dated at 2 weeks and 5 days — and at 2 weeks and 3 days for a day-3 embryo. Nothing has been implanted for a fortnight; the fortnight is a bookkeeping convention that keeps IVF pregnancies on the same scale as everyone else’s, so the anatomy scan window and the growth charts still mean what they mean.

Worked example

Blastocyst transferred on June 20, today August 1:

• Equivalent last menstrual period: June 1, which is the transfer minus 5 days minus 14
• Days elapsed since then: 61
• Gestational age today: 8 weeks and 5 days
• Estimated due date: March 8, which is June 20 plus 261 days
• Trimester: first

Why the scan does not get a vote here

For a spontaneous pregnancy, a first-trimester ultrasound outranks the last menstrual period, because the period is a proxy for ovulation and ovulation moves. Crown-rump length is accurate to plus or minus 5 to 7 days.

IVF removes the uncertainty the scan was there to fix. The fertilization moment was observed in a laboratory, so the ART-derived age is more precise than any measurement of the embryo could be, and ACOG says it is the one used to assign the due date. A dating scan in an IVF pregnancy confirms viability, number of embryos and location — it is not there to renegotiate the date, and a measurement a few days off is expected biological variation, not evidence the clinic was wrong.

Compare that with how dating works when the conception date is unknown, in the gestational age calculator and in gestational age by ultrasound. When two embryos take, the same transfer-date rule applies — see twin gestational age.

This content is informational. Your fertility clinic and prenatal care team hold the dates of record for your pregnancy.