Estimate gestational weeks and an expected due date from the first day of the last period, a clinician-assigned due date, or an IVF embryo transfer. Use the due date recorded by your maternity team for care.
Using IVF dates? In the calculator below, open Dating method and choose IVF embryo transfer date. Then enter the transfer date and select Day-3 embryo or Day-5 embryo from your clinic record. Do not enter the transfer date as the first day of your last period.
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Method and example
For a 28-day cycle, add 280 days to the first day of the last period. If a regular cycle differs from 28 days, adjust the estimate by that difference. For example, a July 22, 2026 last-period date and a 28-day cycle give 10 weeks 0 days on September 30, 2026 and an estimated due date of April 28, 2027.
If your clinician has assigned a due date, gestational age can be counted backward from that date. For IVF, this calculator uses transfer date and the age of a day-3 or day-5 embryo: transfer date + (266 − embryo age in days).
IVF example: For an October 1, 2026 transfer, a day-3 embryo gives an estimated due date of June 21, 2027 (263 days later); a day-5 embryo gives June 19, 2027 (261 days later). These are illustrative dates. Use the embryo age and final due date recorded by your fertility or maternity team.
What the dates mean
Gestational age counts from an obstetric dating start, not the date of fertilization. The due date is an estimate, and an early ultrasound may lead your care team to revise it. An uncertain last period or irregular cycle makes an LMP estimate less reliable.
What to do next
- Compare this result with the final due date in your maternity record.
- Ask your team about differences after an early scan or IVF dating.
- Plan visits and tests with your clinician for your situation.
Frequently asked questions
What if my cycle is not 28 days?
For a regular cycle of 21–35 days, you can enter the usual length and adjust the estimate by the difference from 28 days. Ovulation timing varies, so this is still only an estimate.
What if my ultrasound date differs?
Use the due date your care team has assigned and recorded. An early dating scan can change the estimate calculated from the last period.
Related tools and records
Health records and visit preparation · All calculators
Sources
ACOG–AIUM–SMFM dating guidance · NHS due date calculator · NHS ultrasound dating
Sources reviewed September 30, 2026. General information, not a diagnosis or a substitute for maternity care. No independent clinician review is claimed.