Conception Date Calculator

The 280-day standard (40 weeks) is measured from the first day of the last menstrual period (LMP), not from conception. Because fertilization typically occurs around day 14 of a 28-day cycle, fetal age from conception is approximately 266 days. The LMP is used as the reference point because it is a more reliably known date than the day of ovulation or fertilization.

The estimate has inherent uncertainty of approximately ±2 weeks, reflecting the fertile window and normal variability in ovulation timing. Even with a precise due date from first-trimester ultrasound, the conception window cannot be narrowed below approximately 2 weeks. The calculator provides a best estimate based on standard obstetric assumptions.

For standard pregnancies, conception almost always falls within the calculated range. However, late ovulation (e.g., ovulation on day 21+ in long cycles) could shift the conception date later than the calculator suggests. If the due date was established by early ultrasound (which accounts for delayed ovulation), the estimates are more reliable. If based solely on LMP with a long or irregular cycle, the range may be shifted.

For pregnancies conceived via IVF (in vitro fertilization), the conception date is known precisely — it is the date of embryo transfer (for a 5-day blastocyst transfer, add 5 days back to the fertilization date). IVF due dates are therefore calculated differently: typically 266 days from the egg retrieval date (day 0) or 261 days from a Day 5 transfer. This calculator assumes natural conception and is not appropriate for ART pregnancies.

In clinical practice, the due date drives everything — conception is backward-calculated from it, not the other way around. The EDD is established first (by LMP or early ultrasound), and conception is inferred. Changing the assumed conception date does not change the clinical management of the pregnancy; the EDD established by first-trimester ultrasound is used consistently throughout prenatal care.