Conception Calculator
Estimate the date of conception from your due date or from the first day of your last menstrual period (LMP) — shown as an honest range, not a single pretending-to-be-exact date.
Your EDD — from Naegele's rule (LMP + 280 days) or from an early ultrasound.
Best for roughly 28-day cycles; assumes ovulation around day 14.
Estimated conception
Fertilization ≠ intercourse date: sperm can survive up to about 5 days, so intercourse may have occurred several days before the conception date above. For IVF, the fertilization date is known exactly from the clinic — no estimate needed.
How it works: Naegele's rule, in reverse
Doctors date a pregnancy from the first day of the last menstrual period (LMP), not from conception, because the LMP is observable. The standard rule (Naegele's rule) says:
Ovulation — and therefore fertilization — typically happens about 14 days after the LMP. Subtract those two weeks of "extra" counted time and you get:
Why a range?
Ovulation does not always land exactly on day 14. In typical cycles it falls between day 11 and day 21 after the LMP. Mirrored onto the due-date side, that is due − 269 to due − 259 days. This tool shows that range instead of a single date because a single date would overstate what calendar math can know.
Worked example
Estimated conception: Thursday, January 22, 2026 (2026-10-15 minus 266 days).
Estimated range: January 19 – January 29, 2026 (due minus 269 to minus 259 days).
Check: LMP would be January 8, 2026 (due − 280 days); January 8 + 14 days = January 22. Both modes agree.
Scope boundary
This tool looks backward: it estimates when conception already happened. Looking for something else?
- Already pregnant and want the due date instead? → Pregnancy Due Date Calculator
- Planning ahead and want fertile days? → Ovulation Calculator
Frequently asked questions
What is a conception date?
The conception date is the day an egg was fertilized by sperm. Doctors usually date a pregnancy from the first day of the last menstrual period (LMP) instead, because the LMP is easy to observe while fertilization itself is not.
How does this calculator estimate conception?
It works backward from Naegele's rule. A due date is defined as LMP plus 280 days (40 weeks), and ovulation typically happens about 14 days after the LMP — so conception is estimated as the due date minus 266 days (38 weeks). In LMP mode it is simply the LMP plus 14 days.
Why does it show a range instead of a single date?
Ovulation does not always happen exactly 14 days after the LMP; in typical cycles it falls between day 11 and day 21. The range (due date minus 269 to minus 259 days, or LMP plus 11 to plus 21 days) reflects that natural variation honestly instead of pretending calendar math is exact.
How accurate is the estimate?
It is an educational estimate, not a diagnosis. Cycle length, early or late ovulation, and irregular cycles all shift the real date. Only a first-trimester dating ultrasound, interpreted by your doctor, can pin down gestational age more precisely.
Can I use LMP mode if my cycle is not 28 days?
The LMP estimate assumes ovulation around day 14, which fits a roughly 28-day cycle. If your cycles are much longer, much shorter, or irregular, the estimate is less reliable — the due-date mode is usually better, especially if your due date came from an early ultrasound.
Is the conception date the same as the date of intercourse?
Not necessarily. Sperm can survive in the reproductive tract for up to about 5 days, so the intercourse that led to pregnancy may have happened several days before fertilization actually occurred.
Does this work for IVF pregnancies?
For IVF the fertilization date is known exactly from the lab, so you do not need an estimate — ask your clinic, or subtract the embryo's age in days from the transfer date. This calculator is designed for estimating natural conception.
What is Naegele's rule?
Naegele's rule is the standard method for estimating a due date: add 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. This calculator runs the rule in reverse.
Why might my ultrasound date differ from this estimate?
An early ultrasound measures the embryo directly (crown-rump length) and is more accurate than calendar math. Doctors usually re-date the pregnancy if the ultrasound differs from the LMP-based date by more than about 5 to 7 days in the first trimester.
Is this medical advice?
No. This tool provides educational estimates only and is not medical advice. Always confirm dates and any health decisions with your doctor or midwife.
Free tool by NRTechStudio