Conception Date Calculator
Example: Due date Dec 15 → Conceived around Mar 24
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 for up to 5 days (occasionally 7) 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.
Calculation results updated
Fertile window & conception estimate
Estimated Conception Date
Likely Fertile Window
Pregnancy Status
First Day of Last Period
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How We Calculate This
From due date: conception ≈ due date − 266 days (38 weeks of fetal development, since the 280-day due date counts from the LMP with ovulation assumed on day 14). From LMP: conception ≈ LMP + 14 days, assuming a 28-day cycle. From birth date: conception ≈ birth date − 266 days, which assumes the baby arrived on the due date; births from 37 to 42 weeks are all classed as term, so this mode can be off by up to three weeks either way. For IVF, fertilization is the transfer date minus 5 days for a day-5 embryo or minus 3 days for a day-3 embryo, matching ACOG's 261- and 263-day due-date intervals. The fertile window covers the 5 days before through 1 day after the estimated ovulation day, because sperm survive for up to about 5 days and the egg for about a day.
Methodology last reviewed: September 2026. How SparkCalc works
Sources: ACOG Committee Opinion No. 700: Methods for Estimating the Due Date (reaffirmed 2025) · ACOG Committee Opinion No. 579: Definition of Term Pregnancy · NHS: Periods and fertility in the menstrual cycle
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 for up to 5 days (occasionally 7) in the reproductive tract. The most accurate dating method is a first-trimester ultrasound, up to about 14 weeks, which dates the pregnancy to within about 5 to 7 days and is more precise the earlier it is done.
Why does the calculator add 14 days to LMP?
Ovulation usually happens 10 to 16 days before the next period, which is day 14 in a textbook 28-day cycle. 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.
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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.