Conception Date Calculator

Conception date can be calculated using three methods: 1) From your due date: subtract 266 days (38 weeks), 2) From your last menstrual period (LMP): add 14 days, as ovulation and conception typically occur about 2 weeks after your LMP, or 3) From an ultrasound: your doctor can estimate conception based on fetal measurements. The most accurate method is using an early ultrasound (done before 13 weeks), as it can date pregnancy within 3-5 days of accuracy.

Pregnancy is dated from the first day of your last menstrual period (LMP), but conception doesn't actually occur until about 2 weeks later during ovulation. This is because the exact date of conception is often unknown, but most women can remember their last period. Medical professionals use a 280-day (40-week) pregnancy timeline from LMP, even though actual fetal development is only about 266 days (38 weeks) from conception. This 2-week difference accounts for the time between your period and ovulation/conception.

Conception date calculation is an estimate with a typical accuracy range of +/-3-5 days. The accuracy depends on several factors: regularity of menstrual cycles (women with 28-day cycles get more accurate estimates), the calculation method used (early ultrasound is most accurate), and individual biological variations. For women with irregular cycles, the conception date range may be wider. Sperm can survive in the female reproductive tract for up to 5 days, and the egg is viable for 12-24 hours after ovulation, creating a conception window rather than a single date.

While conception date can provide clues about when pregnancy began, it should not be used alone for paternity determination. The conception window typically spans 5-7 days because sperm can survive in the female body for up to 5 days before fertilizing an egg. This means intercourse that occurred several days before ovulation could still result in conception. For legal or definitive paternity determination, a DNA test is the only reliable method. However, knowing the conception date range can help narrow down the timeframe when conception was possible.