Pregnancy Conception Calculator
Estimate only; check important health decisions with a qualified professional.
2% – 55% reference display (not a diagnosis)
Estimate only; check important health decisions with a qualified professional.
Common questions
When might I have conceived based on my last period?
The calculator estimates a possible conception day using LMP plus cycle length minus 14 days. Ovulation varies, so the date is an approximation.
How does a 30-day cycle change conception timing?
The simplified calendar model places possible ovulation around 16 days after the first day of LMP for a 30-day cycle, approximately two days later than a 28-day-cycle estimate.
What about a 35-day menstrual cycle?
The same model estimates ovulation about 21 days after LMP. Real cycles and ovulation can differ substantially.
Is the conception date the same as the day of sex?
Not necessarily. Sperm can survive for several days, so intercourse and possible fertilization may occur on different dates.
Can a calendar estimate prove the exact conception day?
No. Neither this date nor the displayed fertile window establishes the exact time of fertilization.
Can this tool establish paternity?
No. Calculated dates cannot determine biological parentage. Genetic and qualified medical guidance may be needed.
Can I use the fertile window as contraception?
No. This calculator is not a birth-control method and should not be used to identify supposedly pregnancy-free days.
Can I calculate from my due date, ultrasound or IVF transfer here?
Not in this version. The available inputs are LMP and typical cycle length. A clinician can interpret ultrasound or assisted-reproduction dating records.
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Learn more
How to Estimate a Conception Date From Your Last Period
Enter the first day of the last menstrual period (LMP) and your typical cycle length. The result estimates a possible fertilization date and a broad fertile window. The calculator also shows a corresponding estimated due date. These are educational calendar estimates, not exact biological dates.
Conception Date Formula and Examples
The simple estimate used here is conception date ≈ LMP + (cycle length − 14) days. With a 28-day cycle, that is about 14 days after LMP; a 30-day cycle gives about 16 days; a 35-day cycle gives about 21 days. Ovulation is not guaranteed on these dates, and variation is common.
For an LMP of 1 January 2026 with a 28-day cycle, this estimate places conception near 15 January 2026 and the estimated due date near 8 October 2026. The displayed fertile window stretches around the calculated conception date because intercourse and fertilization are not necessarily on the same day.
Conception Date Versus Intercourse Date
Fertilization usually occurs around ovulation. Sperm may remain viable for several days beforehand, so a calculation cannot identify one act of intercourse, verify paternity or establish precisely when pregnancy began. Irregular cycles add additional uncertainty.
This tool estimates possible conception timing from LMP, not pregnancy week-by-week age. For that use the Pregnancy Calculator; to estimate delivery from LMP or a conception date, use the Due Date Calculator.
References: ACOG — Fertility Awareness-Based Methods; ACOG — Pregnancy Dating. Do not rely on this estimate for contraception, diagnosis or paternity decisions.