Due date formulas: how we calculate
These are the exact formulas. Each one comes from a published medical source.
A pregnancy is counted from the first day of your last period, not from conception. A typical pregnancy lasts 280 days, or 40 weeks, from that day.
From your last period (Naegele’s rule)
Due date = first day of your last period + 280 days. Example: a last period on August 7, 2026 gives a due date of May 14, 2027.
Adjusting for your cycle length
A 28-day cycle is the standard. For longer or shorter cycles, we add or remove the difference. Example: a 35-day cycle adds 7 days, so August 7, 2026 gives May 21, 2027. If your cycles are irregular, we show a range for cycles of 21 to 35 days.
From conception
Due date = conception date + 266 days. Example: conception on August 21, 2026 gives May 14, 2027.
From an IVF transfer
Due date = transfer date + 261 days for a day-5 embryo, or + 263 days for a day-3 embryo. Example: a day-5 transfer on March 1, 2026 gives November 17, 2026; a day-3 transfer on the same day gives November 19, 2026.
From an ultrasound
Due date = scan date + (280 days − the age measured at the scan). Example: a scan on November 10, 2026 measuring 12 weeks 3 days gives May 22, 2027.
Ultrasound or last period: which date wins?
Doctors switch to the ultrasound date only when the two dates differ by more than a limit. The limit grows as the pregnancy goes on, because later scans are less exact.
| Age at the scan | Use the scan if the dates differ by more than |
|---|---|
| 0w 0d – 8w 6d | 5 days |
| 9w 0d – 13w 6d | 7 days |
| 14w 0d – 15w 6d | 7 days |
| 16w 0d – 21w 6d | 10 days |
| 22w 0d – 27w 6d | 14 days |
| 28w 0d and later | 21 days |
The 37 to 42 week range
Babies born from 37 weeks 0 days are “term”. Early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and post-term is 42 weeks or later (ACOG). We show the range from 37 weeks to 42 weeks.
Twins and more
Twins usually come earlier: around 6 in 10 twins are born before 37 weeks (NHS). Your care team will plan the timing of the birth with you. When you tick “twins or more”, we still show the due date, but we hide the birth chances and the baby’s weight, because those figures are for single babies.
When babies really arrive
Our chances come from official US birth statistics for 2022 (CDC): 11.6% of single babies were born at 37 weeks, 17.5% at 38, 38.8% at 39, 18.3% at 40, 4.9% at 41 and 0.3% at 42 weeks or later; 8.7% were born before 37 weeks. The statistics are by week, so we spread each week evenly over its 7 days. Practice differs between countries, for example in how often labour is induced.
Your baby’s weight
From 14 to 40 weeks we show the average (50th percentile) estimated weight from the WHO fetal growth charts (2017), a study of healthy single pregnancies in 10 countries. We compare it only with everyday things whose weight is printed on the label, and only when they are within 15% of the weight. We do not show it for twins.
Exact calendar days
All dates are counted in whole calendar days. Time zones and daylight-saving changes can never move your date by one day.
Sources
- Methods for Estimating the Due Date (Committee Opinion No. 700) · ACOG · checked October 24, 2026
- Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign (Committee Opinion No. 651) · ACOG · checked October 24, 2026
- Definition of Term Pregnancy (Committee Opinion No. 579) · ACOG / SMFM · checked October 24, 2026
- Shifts in the Distribution of Births by Gestational Age: United States, 2014–2022 · CDC / NCHS · checked October 24, 2026
- Multiple pregnancy and screening for preterm birth · NHS (Chelsea and Westminster Hospital) · checked October 24, 2026
- The World Health Organization Fetal Growth Charts · WHO · checked October 24, 2026
- Timing of Sexual Intercourse in Relation to Ovulation · NEJM · checked October 24, 2026