Conception Date Calculator
Estimate when conception occurred by working backwards from your due date or birth date.
Calculate conception date range, first trimester start, and ultrasound date comparison.
How Conception Date Is Calculated Standard pregnancy duration is 40 weeks (280 days) from the last menstrual period (LMP), not from conception. Conception typically occurs about 2 weeks after the LMP during ovulation. So conception date ≈ Due date − 38 weeks (266 days). Obstetricians use the LMP as the reference point because it is easier to track than the actual fertilization date.
Why There Is a Range Sperm can survive in the reproductive tract for 3–5 days after intercourse. The egg survives for 12–24 hours after ovulation. Ovulation can vary by ±2–3 days from cycle to cycle. This means the actual conception window spans approximately 5–7 days around estimated ovulation.
Naegele’s Rule (Due Date Formula) The standard formula for estimating due date from LMP: Due Date = LMP + 9 months + 7 days (or equivalently, LMP + 280 days). Named after Franz Karl Naegele, a German obstetrician who published the method in 1830 in Germany. This assumes a regular 28-day cycle. Adjustments are made for longer or shorter cycles. Ultrasound dating in the first trimester (8–13 weeks) is more accurate than LMP dating and is used when there is a discrepancy.
First Trimester Development Timeline Week 1–2: The pre-embryonic period. No conception yet — the body prepares for ovulation. Week 3: Fertilization and implantation. The fertilized egg becomes a blastocyst. Week 4: The embryo implants in the uterine wall. hCG hormone begins rising — home tests can detect pregnancy. Week 5–6: The heart begins beating. The neural tube (brain and spinal cord) forms. Week 8: All major organs are forming. The embryo is now called a fetus. Week 10: Fingers, toes, and facial features are visible on ultrasound. Week 12: End of first trimester. Miscarriage risk drops significantly. Nuchal translucency scan. Week 20: Mid-pregnancy anatomy scan. Sex determination is usually possible.
Accuracy of Conception Date Estimates LMP-based estimates have a margin of error of ±2 weeks. Ultrasound dating at 8–12 weeks is accurate to ±5–7 days. Ultrasound at 18–20 weeks has a margin of ±10–14 days. DNA paternity testing can confirm biological parenthood but cannot narrow conception to a specific day. No method can pinpoint an exact conception date — all results are estimates.