Conception Calculator

For natural pregnancies: conception happens around ovulation, which is approximately 14 days after the first day of your last period (for a 28-day cycle). The formula: conception date = LMP + (cycle length − 14). If you only know your due date, conception date ≈ due date − 266 days.

Subtract 266 days (38 weeks) from your estimated due date. For example, a December 1 due date means conception happened around March 10. The actual conception occurred within a 6-day fertile window around that date.

Not exactly — conception happens at fertilisation, which is within 24 hours of ovulation. Sperm can survive 5 days, so the 'when' is technically a 5-day window. The most likely day is on or one day after ovulation. For paternity questions, a 10-14 day window is typically used.

For regular 28-day cycles with known LMP, accurate within ±3 days. Irregular cycles introduce more variation. Early ultrasound (weeks 7-13) is the most accurate way to confirm dating.

Conception (fertilisation) happens within 24 hours of ovulation. Implantation — when the fertilised egg attaches to the uterine lining — happens 6-12 days later. Pregnancy tests detect hCG, which is produced after implantation, not conception.

Because pregnancy is counted from the first day of your last period, not from conception. Ovulation/conception happens about 14 days after LMP. So 280 (LMP-based) − 14 (LMP to conception) = 266 days from conception to delivery.