How conception dating works
A due date is set 280 days after the last period, and ovulation is assumed about 14 days after that period starts. So conception is roughly due date − 266 days. If you enter your period instead, we place ovulation 14 days before your next expected period, which handles longer and shorter cycles.
Research by Wilcox and colleagues (NEJM, 1995) found pregnancy only resulted from sex in a six-day window ending on the day of ovulation. That’s why the calculator shows a range, not a single day. The calculator can’t prove when conception happened and shouldn’t be used to settle paternity questions.
Questions people ask
How do I calculate my conception date?
Count back 266 days (38 weeks) from your due date, or forward from your last period to about 14 days before your next expected period. The result is the most likely ovulation and conception day.
Can a conception calculator tell who the father is?
No. Sperm can survive up to five days, ovulation timing varies from cycle to cycle, and due dates are themselves estimates, so the window is at least a week wide. Only a DNA test can establish paternity.
Is conception the same day as intercourse?
Not necessarily. Because sperm can live for several days, conception can happen up to about five days after sex, once an egg is released.
How accurate is it after an ultrasound?
If your due date comes from a first-trimester ultrasound, it is usually accurate to within about 5–7 days, which makes the conception estimate better, but never exact.
Sources
- ACOG Committee Opinion No. 700. EDD from conception = + 266 days; accuracy of first-trimester ultrasound dating.
- Wilcox AJ et al. (1995). Timing of sexual intercourse in relation to ovulation. N Engl J Med 333:1517–1521 (six-day fertile window ending on ovulation day).
- MedlinePlus. Identifying fertile days; sperm survival up to five days.
Every formula on VitalMetrics is listed with its original reference on Sources & methods.