Medically reviewed by Samantha L. Fox, RN, BSN, MSN
Nursing Informaticist · United States Navy Nurse Corps · Last reviewed
Conception Date Calculator
Just had an ultrasound? Enter the date and gestational age it measured to estimate both your due date and when conception likely occurred. Or work backward from a known due date.
Choose your input
Look on your ultrasound report for something like “8w3d” or “EGA 8 weeks 3 days.”
How the Conception Date Calculator Works
From an ultrasound (most accurate)
When you have an early ultrasound, the technician measures your baby and assigns a gestational age — typically expressed as weeks and days (e.g. “8 weeks 3 days” or “8w3d”). That measurement tells us how far along the pregnancy is on the day of the scan. From there, we work both directions: forward to estimate your due date (full pregnancy is 280 days from the conventional “start” — the first day of your last menstrual period), and backward to estimate when conception happened (266 days before the due date).
This is especially useful when a pregnancy is unexpected. Many women find out they're pregnant during an ultrasound for an unrelated reason, or after a positive test makes them realize their period was late. Knowing the conception window helps make sense of the timeline — what month it happened, who was around, what was going on in life.
⚠️ A note on accuracy from our clinical reviewer:Babies don't all grow at the same rate. Early in the first trimester, embryos develop on a remarkably consistent schedule, which is why an ultrasound at 6-12 weeks is the gold standard for dating. After 13-14 weeks, individual variation in fetal growth widens — by the third trimester, an ultrasound's estimated gestational age can be off by 2-3 weeks. The earlier the ultrasound, the more reliable the conception date estimate.
From a due date
Already have a due date from your provider? We subtract 266 days (38 weeks) to estimate the conception date. This works because pregnancy lasts approximately 38 weeks from actual conception, even though it's measured as 40 weeks from your last menstrual period. The 2-week difference accounts for the time between your period starting and ovulation actually occurring.
Why we show a window, not a single day
We display a likely conception window of about 6 days because biology has natural uncertainty. Sperm can survive in the female reproductive tract for up to 5 days, and the egg survives for 12-24 hours after ovulation. So intercourse anywhere in that roughly 6-day window could have led to pregnancy. The conception “date” is the day fertilization most likely occurred — usually within a day of ovulation.
For the most reliable conception date estimate, use the results from a first-trimester ultrasound (before 13 weeks 6 days). If your ultrasound was later, treat the conception date as a rough window of ±1-2 weeks rather than a precise day, and bring your ultrasound report to your prenatal appointment if you have specific questions.
📚Clinical Sources & References
The calculations and guidance on this page are based on current clinical standards and peer-reviewed research. Reviewed by Samantha L. Fox, RN, BSN, MSN — Emergency Department nurse and US Navy Nurse Corps officer.
- [1]Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333(23):1517-1521.
- [2]ACOG Practice Bulletin No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
- [3]Stanford JB, White GL, Hatasaka H. Timing intercourse to achieve pregnancy: current evidence. Obstet Gynecol. 2002;100(6):1333-1341.
- [4]Dunson DB, Colombo B, Baird DD. Changes with age in the level and duration of fertility in the menstrual cycle. Hum Reprod. 2002;17(5):1399-1403.