Pregnancy Conception Calculator

How the conception date calculator works

This calculator follows the same approach as Calculator.net by working backward from a due date or forward from your last menstrual period (LMP). A typical pregnancy is measured at 40 weeks (280 days) from LMP, while conception usually happens about 2 weeks later. That difference is why gestational age and fetal age are not the same.

If you start with a due date

• We estimate the LMP date by subtracting 280 days from your due date.
• Estimated ovulation is set at 266 days before the due date (38 weeks), which is the average time from conception to birth.
• The conception window covers the 5-day fertile window around ovulation.

If you start with LMP + cycle length

• We estimate ovulation by subtracting 14 days from your average cycle length.
• The due date is calculated by adding 280 days, then adjusting for longer or shorter cycles.
• The conception window is centered around the ovulation date to reflect peak fertility.

Understanding gestational age vs. fetal age

Gestational age counts from the first day of your last period. Fetal age starts around two weeks later, near the time of conception. Clinicians use gestational age for most pregnancy milestones, so we report both for clarity.

These dates are estimates and can vary with cycle irregularities or ultrasound findings. Always confirm timing with a clinician, especially if you have irregular cycles or assisted reproduction.