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Pregnancy Conception Calculator

Estimate the likely date of conception based on the due date or birth date.

Your details

Results

Estimated conception date
Apr 28, 2026
LMP ≈ Apr 14, 2026
Estimated LMP
Apr 14, 2026
Conception − 14 days
Ovulation window
Apr 26, 2026 – Apr 30, 2026
Most fertile days
Current gestational age
20w 0d
Day 140 of 280
Pregnancy progress
50%
Timeline
EventDateDays from conception
Last menstrual periodApr 14, 2026−14
Ovulation window startsApr 26, 2026−2
Conception (estimate)Apr 28, 20260
Ovulation window endsApr 30, 2026+2
Estimated due dateJan 19, 2027+266
Conception is back-calculated as due date − 266 days(38 weeks gestation from conception). Sperm can live up to 5 days inside the body and the egg survives about 24 hours after ovulation, so conception may have occurred on any day in the fertile window shown above. Estimates are approximate.

What is the Pregnancy Conception Calculator?

A pregnancy conception calculator estimates the date your baby was likely conceived based on your due date, last menstrual period, or birth date. Because ovulation usually happens about 14 days before the next period, conception is estimated around that window.

Pregnancy Conception Calculator formula

Conception date ≈ LMP + 14 days (for a 28-day cycle). If you only know the due date: Conception ≈ Due Date − 38 weeks (because the due date is LMP + 40 weeks, and conception is ~LMP + 2 weeks). For cycles shorter or longer than 28 days, adjust by (cycle length − 28) days.

How to use the Conception calculator

  1. 1Enter either your due date, the first day of your last period, or your baby's birth date.
  2. 2Optionally enter your average menstrual cycle length.
  3. 3Click Calculate to see the estimated conception date and fertile window.

Example calculation

If your due date is October 10, the estimated conception date is October 10 − 38 weeks = approximately January 25. The fertile window would have been around January 20–27, with ovulation most likely on or near January 25.

Frequently asked questions

How do I calculate my conception date?
If you know your due date, subtract 38 weeks. If you know your last menstrual period, add about 14 days (for a 28-day cycle). For longer or shorter cycles, adjust by (cycle length − 28) days. Ultrasound dating gives the most precise estimate.
Can the conception date be wrong?
Yes — sperm can survive up to 5 days in the reproductive tract and ovulation timing varies, so conception can occur within a several-day window. The calculator provides an estimate, not an exact date.
What is the fertile window?
The fertile window is the 5 days before ovulation plus the day of ovulation itself — about 6 days each cycle. Conception is most likely when intercourse happens in this window.
How accurate is conception dating from LMP?
For women with regular 28-day cycles, LMP-based conception dating is reasonably close. For irregular cycles, an early first-trimester ultrasound is far more accurate. Consult your healthcare provider for personalized dating.

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