Estimate an approximate conception date, due date, ovulation date, and likely fertile window from the pregnancy date you already know.
Advanced options
This only changes how dates are shown, not the math.
How to use our Conception Calculator
- Choose your starting point in "What do you know?" and enter the matching date: "Estimated due date," "First day of last menstrual period," "Ovulation date," or "IVF transfer date."
- If you use "First day of last menstrual period," enter "Average cycle length (days)" as a whole number; if you use IVF, choose "Embryo age at transfer (days)."
- Pick your preferred "Date display style" if needed, then click "Calculate."
- Read "Estimated conception date" as the best single-date estimate, then compare it with "Likely fertile window" to see the wider range when intercourse could have led to pregnancy.
- Sanity-check the result by making sure the dates follow a logical order: "Pregnancy dating start (LMP-based)" should come before "Estimated ovulation date," and "Estimated due date" should be later than both.
Definitions
Estimated conception date: The best single-date estimate for when pregnancy likely began based on the method you selected.
Estimated due date: The expected delivery date calculated from your selected starting date.
Estimated ovulation date: The ovulation date used in the math. It may be entered directly or estimated from your cycle.
Likely fertile window: A date range around ovulation when intercourse could have led to pregnancy. This is wider than the conception date.
Pregnancy dating start (LMP-based): The day counted as week 0 in standard pregnancy dating, usually the first day of the last menstrual period.
Average cycle length (days): The typical number of days from the first day of one period to the first day of the next period.
Embryo age at transfer (days): In IVF, the number of days the embryo had developed before transfer, such as 3 or 5.
Method note: A short explanation of the rule used for your estimate and why it may differ from another method.
Common mistakes and quick fixes
Mistake: Entering the last day of bleeding in "First day of last menstrual period".
Fix: Use day 1 of bleeding for "First day of last menstrual period," because pregnancy dating starts from that date.
Mistake: Leaving "Average cycle length (days)" blank in LMP mode or entering 0.
Fix: Enter your usual whole-number cycle length, such as 28, so "Estimated ovulation date" and "Estimated conception date" can be calculated correctly.
Mistake: Using an intercourse date as "Estimated conception date".
Fix: Treat "Estimated conception date" as an estimate near ovulation, and use "Likely fertile window" for the broader time when intercourse may have led to pregnancy.
Mistake: Picking IVF mode but forgetting "Embryo age at transfer (days)".
Fix: Choose the correct transfer type, such as day 3 or day 5, because it changes both "Estimated conception date" and "Estimated due date."
Mistake: Comparing dates from different methods as if they must match exactly.
Fix: Read "Method note" to see which rule was used, because due date, LMP, ovulation tracking, and IVF records can give slightly different estimates.
Mistake: Entering an unusually short or long cycle in "Average cycle length (days)".
Fix: Use a whole number in the allowed range only, because out-of-range values make the LMP-based estimate less reliable.
Limitations & Key Assumptions / Boundary Conditions
- All results are estimates based on the dates you enter and simple dating rules, not proof of the exact day pregnancy began.
- LMP mode assumes ovulation happens about 14 days before the next period, so it works best for fairly regular cycles.
- "Average cycle length (days)" must be a whole number in the allowed range; very irregular cycles can make LMP-based results less reliable.
- "Likely fertile window" is a practical range tied to ovulation, not a guarantee that conception happened on any one day in that range.
- Intercourse date and conception date are not the same thing. Sperm can survive for several days before ovulation, and the egg remains viable briefly after ovulation.
- Due dates can differ from clinical dating based on ultrasound, treatment records, or provider judgment.
- Future dates can still be calculated, but the tool only applies the date rules to what you entered and does not judge whether the situation is clinically realistic.
Methodology
How the calculator works
The calculator uses the starting date you know best, then applies standard calendar-day offsets to estimate the other pregnancy dates. It uses real date arithmetic, so leap years and different month lengths are handled automatically.
Formulas used
conception_date = due_date - 266 days
due_date = ovulation_date + 266 days
ovulation_date = lmp_date + (cycle_length - 14) days
conception_date = lmp_date + (cycle_length - 14) days
due_date = lmp_date + cycle_length + 252 days
conception_date = transfer_date - embryo_age days
due_date = transfer_date + (266 - embryo_age) days
fertile_window_start = ovulation_date - 5 days
fertile_window_end = ovulation_date + 1 day
What each mode means
If you choose due date mode, the tool counts back 266 days to estimate conception, then uses that same date as the estimated ovulation date. The LMP-based pregnancy dating start is set 14 days before the estimated ovulation date.
If you choose LMP mode, the tool estimates ovulation from the first day of the last period plus cycle length minus 14 days. That estimated ovulation date is also used as the estimated conception date, and the due date is set 266 days after ovulation.
If you choose ovulation mode, the entered ovulation date is used as the estimated conception date for dating purposes, and the due date is 266 days later. The LMP-based pregnancy dating start is set 14 days earlier.
If you choose IVF transfer mode, the calculator subtracts the embryo age at transfer to estimate conception, then adds the remaining days up to 266 to estimate the due date. The LMP-based pregnancy dating start is set 14 days before the estimated conception date.
Worked mini-example
Example: in LMP mode, if "First day of last menstrual period" is January 1, 2026 and "Average cycle length (days)" is 28, then estimated ovulation is January 15, 2026 because 28 minus 14 equals 14 days after LMP. The estimated conception date is also January 15, 2026, and the estimated due date is October 8, 2026.
Assumptions behind the estimates
The tool uses a 266-day average from conception to due date and a 14-day luteal-phase assumption for simple LMP-based dating. The fertile window is shown as 5 days before ovulation through 1 day after ovulation. Real pregnancies can vary, and even when ovulation is known, pregnancy length is not exactly the same for everyone [3].