How the due date is calculated
Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception. A typical pregnancy is 280 days, or 40 weeks, from that day. This is called Naegele's rule.
From conception: due date = conception + 266 days
IVF, day-5 embryo: transfer date + 261 days
IVF, day-3 embryo: transfer date + 263 days
From ultrasound: scan date + (280 − gestational age on the scan, in days)
The cycle adjustment assumes ovulation happens about 14 days before the next period. With a 35-day cycle, ovulation is about a week later than in a 28-day cycle, so the due date moves a week later too.
Dates are counted as whole calendar days, so daylight saving time changes never shift the answer.
Worked example
Your last period started on January 1, 2026, and your cycle is 28 days. Add 280 days: January 1 + 280 days = October 8, 2026. If your cycle is 35 days, add 7 more days: October 15, 2026.
From an IVF transfer of a day-5 embryo on February 10, 2026: February 10 + 261 days = October 29, 2026. The embryo was already 5 days old, and 14 days are added for the weeks counted before conception, so 280 − 14 − 5 = 261.
Trimesters and milestones
| Stage | Weeks |
|---|---|
| First trimester | 0 weeks to 13 weeks 6 days |
| Second trimester | 14 weeks 0 days to 27 weeks 6 days |
| Third trimester | 28 weeks 0 days to birth |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks 0 days and later |
When this estimate may not apply
- Ultrasound dating may differ. ACOG advises that an ultrasound in the first trimester is the most accurate way to set or confirm the due date. If it differs enough from your LMP date, your clinician will use the ultrasound date.
- Irregular cycles. If your cycles vary a lot, or you aren't sure of your LMP, the LMP date is less reliable.
- Recent hormonal birth control or breastfeeding can make the timing of ovulation hard to know.
- Twins and other multiples often arrive earlier than the due date.
Only about 5% of babies are born on their due date. A large Australian study found this whatever method was used to set the date.
Sources
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date (2017) LMP, ultrasound and IVF (261 and 263 days) dating.
- ACOG Committee Opinion No. 579: Definition of Term Pregnancy early, full, late and post-term.
- Khambalia AZ et al. Predicting date of birth: the best time to date a pregnancy? Int J Gynaecol Obstet. 2013;123(2):105–109 about 5% of births on the estimated date.