Conception Date Calculator
Work backwards from your due date, last menstrual period (LMP), or baby's birth date to estimate when conception likely occurred. Because sperm can survive up to 5 days and ovulation timing varies between cycles, the conception date is always a window rather than a single day. This calculator shows the most likely day and the surrounding fertile window, which is useful for new parents reviewing the pregnancy timeline, planning future pregnancies, or thinking through an IVF timeline.
How We Calculate This
From due date: conception ≈ due date − 266 days (38 weeks of fetal development). From LMP: conception ≈ LMP + 14 days, assuming a 28-day cycle. From birth date: conception ≈ birth date − 266 days. The fertile window covers the 5 days before through 1 day after the estimated ovulation day.
Frequently Asked Questions
How accurate is a conception date estimate?
Conception dates from these formulas are always estimates. Ovulation can vary by several days between cycles, and sperm can survive 3-5 days in the reproductive tract. The most accurate dating method is an early ultrasound between weeks 8 and 13, which is usually within a few days of the true conception date.
Why does the calculator add 14 days to LMP?
In a textbook 28-day cycle, ovulation happens around day 14. Naegele's rule assumes this when working forwards or backwards from the last menstrual period. If your cycle is longer or shorter than 28 days, the conception date shifts by roughly the same number of days as the difference.
Can I use this to figure out paternity?
This calculator gives a fairly wide window and is not a paternity tool. An early ultrasound dates the pregnancy within a few days. For a definitive answer, non-invasive prenatal paternity tests are available after about 7 weeks of pregnancy, and standard DNA testing is available after birth.
How does IVF change the calculation?
For IVF, the fertilization date is the day of egg retrieval. If you had a day-5 blastocyst transfer, fertilization happened 5 days before the transfer; for a day-3 transfer, 3 days before. Use the egg retrieval date as your conception date for the most accurate result.
Related Calculators
You might also find these calculators helpful: Due Date Calculator, Child Growth Percentile, and Age Calculator.
This calculator provides estimates only. Actual conception dates can only be confirmed with medical assessments such as early ultrasound dating. For pregnancy, fertility, or paternity decisions, consult a qualified healthcare provider.