Pregnancy Due Date Calculator — Due Date from Conception Date
For those who tracked ovulation carefully, conceived through assisted insemination, or had an IVF embryo transfer. Enter the date intercourse or insemination occurred (or, for IVF, the transfer date and embryo day) and the calculator works forward: a known conception on January 15, 2026, adds 266 days to arrive at an estimated due date of October 8, 2026. A day-5 blastocyst transfer on February 1, 2026, adds 261 days, giving October 20, 2026. Because conception dating skips the two-week offset baked into Naegele's rule, it is often more accurate when the conception date is reliably known — particularly for IVF pregnancies where embryo age is documented to the exact day. This estimate is still a statistical average, not a guarantee of delivery on a specific date. About 4% of births fall on the exact predicted day, and full term spans 37–42 weeks. IVF dating is confirmed or refined by the fertility clinic and subsequent ultrasounds. As with all due date methods, your prenatal care provider's clinical assessment is the authoritative source for your individual pregnancy.
Estimated due date
Estimated due date
September 24, 2026
40 weeks from December 18, 2025 (LMP)
As of today
Uses Naegele's rule — a 40-week (280-day) estimate from the first day of your last period; conception dating subtracts the ~14-day luteal phase, and IVF dating counts back from the embryo's age. Only about 4% of babies arrive on the exact due date. This is an estimate, not a medical diagnosis or a dating ultrasound — confirm with your provider.
Why conception-based dating is often more precise for IVF
In a natural conception, the exact date of fertilisation is rarely known — even careful ovulation tracking leaves a window of a day or two. But in IVF, every day of embryo development is documented. A day-5 blastocyst transferred on February 1, 2026 was fertilised five days earlier (January 27, 2026), and the due date is 266 days after that fertilisation date — which is the same as adding 261 days to the transfer date itself, giving October 20, 2026. A day-3 transfer works the same way, adding 263 days: the embryo was fertilised three days before transfer, so two fewer days are added to the transfer date. The precision matters because IVF embryos are age-matched to the calendar in a way natural cycles usually are not.
Fertility clinics generate their own due date estimate from the transfer date, and that figure is typically carried forward into the obstetric record. A first-trimester ultrasound may then confirm the estimate or make a small adjustment. Because IVF-conceived pregnancies are closely monitored from the start, the due date is generally more reliable than one derived from an uncertain LMP — but the same 37–42 week full-term window still applies, and most IVF pregnancies are managed to delivery at or before 40 weeks based on the clinical picture.
Natural conception — when you know the conception date
If you tracked ovulation with basal body temperature, LH surge testing, or confirmed timing through another method, you can bypass the LMP altogether. The formula is straightforward: add 266 days to the conception date. A confirmed ovulation and conception on January 15, 2026, places the estimated due date at October 8, 2026 — identical to the Naegele result for an LMP of January 1, 2026, because 14 days + 266 days = 280 days. The two methods converge when the LMP assumption about ovulation timing is correct.
The conception-date method becomes more useful than the LMP method when cycle length differs from 28 days. If ovulation happened on cycle day 21 and you know that date, adding 266 days directly corrects for the longer cycle without any adjustment step. The result is the same estimate your provider would reach after manually correcting Naegele's rule for a 35-day cycle, but arrived at from first principles. Whichever method gives the best-known anchor date — LMP or conception — is the one to use as a starting point before the dating ultrasound.
Questions
- What is the due date if conception occurred on January 15, 2026?
- October 8, 2026. The conception-date method adds 266 days (38 weeks) to the conception date. January 15 plus 266 days lands on October 8, 2026 — the same date Naegele's rule returns for an LMP of January 1, 2026, because the two methods are mathematically equivalent when ovulation falls exactly on cycle day 14.
- What is the due date for a day-5 IVF transfer on February 1, 2026?
- October 20, 2026. A day-5 blastocyst transfer adds 261 days to the transfer date: the embryo was fertilised five days before transfer (January 27, 2026), and 266 days after fertilisation lands on October 20, 2026. For a day-3 transfer on the same date, the calculator would add 263 days, giving October 22, 2026, because the embryo was fertilised only three days earlier.
- Does the conception-date method replace a dating ultrasound?
- No — even a precisely known conception date is a starting estimate. Fetal growth in early pregnancy can vary, and a first-trimester ultrasound remains the most accurate way to confirm gestational age. For IVF pregnancies the fertility clinic's dating is carried forward into the obstetric record, but the obstetrician or midwife will still use ultrasound measurements and clinical findings to monitor progress. This calculator is a planning tool, not a clinical instrument.