Why the methods use different day-counts
A full-term pregnancy is measured as about 280 days (40 weeks) from the first day of the last menstrual period, but only about 266 days (38 weeks) from conception. The 14-day gap is the assumed time between the start of the last period and ovulation. Each dating method starts its count from a different event, so each adds a different number of days to reach the same estimated due date.
The three methods and their day-counts
The table shows what each method starts from and how many days it adds. IVF dating is split by transfer day because the embryo's age at transfer is known precisely.
| Method | Starts from | Days added |
|---|---|---|
| Naegele's rule (LMP) | First day of last period | 280 days (40 weeks) |
| Conception / ovulation | Date of conception | 266 days (38 weeks) |
| IVF Day-5 (blastocyst) | Embryo transfer date | 261 days |
| IVF Day-3 | Embryo transfer date | 263 days |
Why LMP is common but least precise
Naegele's rule is the default because the last period is usually the one date a person can report, but it assumes a textbook 28-day cycle with ovulation on day 14. Cycles that are longer, shorter or irregular shift the true conception date away from that assumption, so the LMP estimate can be off by days in either direction. This is why an early ultrasound, which measures the embryo directly, is often used to confirm or adjust the LMP-based due date.
Why IVF dating is the most accurate
With IVF, the date of fertilisation and the embryo's age at transfer are both known, so there is no assumption about cycle length or ovulation timing. A Day-5 blastocyst is five days past fertilisation at transfer, so 261 days are added (266 minus the 5 days already elapsed); a Day-3 embryo has 263 days added. This removes the largest source of uncertainty in the other methods. Regardless of method, a due date is an estimate of a roughly two-week window, not a fixed appointment — only about 1 in 20 births occurs exactly on the estimated date.
الأسئلة الشائعة
How is a due date calculated from the last period?
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a regular 28-day cycle with ovulation on day 14.
Why is the conception due date 266 days, not 280?
The 280-day LMP count includes about 14 days before ovulation. Dating from conception itself removes that gap, so it adds 266 days (38 weeks).
How is an IVF due date calculated?
IVF dating is exact because the embryo's age is known. A Day-5 blastocyst transfer adds 261 days to the transfer date; a Day-3 transfer adds 263 days.
Which dating method is most accurate?
IVF dating is the most precise because fertilisation timing is known. LMP is the most common but least precise, and early ultrasound is often used to confirm it.
المراجع
- American College of Obstetricians and Gynecologists (ACOG) Committee Opinion 700 — Methods for Estimating the Due Date. https://www.acog.org/
- National Health Service (UK) — working out your due date. https://www.nhs.uk/pregnancy/
- American Society for Reproductive Medicine (ASRM) — IVF and embryo transfer dating. https://www.asrm.org/