Estimate the most likely conception date and fertile window from your last period and average cycle length.
Most likely conception date
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-Estimates assume a regular cycle with ovulation ~14 days before the next period. Actual conception can vary; sperm can survive up to ~5 days, so conception may occur a few days after intercourse. Not a substitute for medical advice or testing.
Example
If your last period started on 2026-05-01 and your average cycle is 30 days, ovulation is estimated at LMP + (30 - 14) = 16 days later:
LMP: 2026-05-01
Cycle length: 30 days
Ovulation: 2026-05-01 + 16 = 2026-05-17
Fertile window: 2026-05-12 to 2026-05-17
Most likely conception: 2026-05-17
Estimated due date: 2026-05-17 + 266 = 2027-02-07
How it works
Ovulation is estimated as the LMP date plus (cycle length minus 14 days), since the luteal phase is consistently about 14 days. The fertile window spans the 5 days before ovulation through ovulation day, and the most likely conception date is ovulation day.
Good to know
The Conception Calculator works backward from a known cycle to pinpoint when conception most likely happened. You enter the first day of your last menstrual period (LMP) and your average cycle length, and it returns your estimated ovulation day, a fertile window, the single most likely conception date, and an estimated due date counted 266 days from ovulation. It is built for anyone who already suspects or knows they are pregnant and wants to map the timeline, as well as those tracking cycles to understand when conception is most probable.
People typically reach for this after a positive test, when comparing a conception estimate against an LMP-based due date, or when trying to align a likely conception date with specific dates of intercourse. Because the calculator places ovulation at LMP plus (cycle length minus 14), a longer or shorter cycle shifts every result accordingly, so entering your real average rather than the default 28 days matters a great deal.
Read the output as a range, not a verdict. The bold "most likely conception date" is ovulation day itself, but the fertile window covers the five days before it because sperm can survive that long, meaning conception could occur up to several days after intercourse. The fertile-window start and end dates tell you the realistic span during which conception was possible.
- If your cycles are irregular, the fixed 14-day luteal assumption becomes less reliable and the window is your better guide than the single date.
- An ultrasound dating scan generally overrides any calculator estimate, since early measurements reflect actual fetal growth rather than cycle math.
Frequently asked questions
How is the conception date calculated?
Conception is estimated to occur at ovulation, which this tool places at your LMP date plus (cycle length minus 14), because the luteal phase after ovulation is consistently around 14 days regardless of cycle length.
Why is there a fertile window instead of a single day?
Sperm can survive in the reproductive tract for up to about 5 days, so intercourse anytime from 5 days before ovulation through ovulation day can lead to conception. The single 'most likely' date is ovulation itself.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is this a substitute for medical advice?
No. These are educational estimates — consult a qualified health professional for medical decisions.
People also ask
What is the difference between conception date and gestational age?
Gestational age is counted from the first day of your last period, while conception date is roughly two weeks later at ovulation. This means a pregnancy is typically about two weeks 'pregnant' on paper before conception actually occurs.
Can a conception calculator tell me exactly who the father is?
No. It only estimates a likely range of dates based on cycle math, and the fertile window spans several days, so it cannot confirm paternity. DNA testing is the only reliable way to establish biological parentage.
How accurate is estimating conception from the last menstrual period?
Accuracy depends on having regular cycles and knowing your true average cycle length, since the tool assumes ovulation occurs about 14 days before the next period. Irregular cycles, late or early ovulation, and uncertain LMP dates all reduce accuracy.
Why does the due date come out 266 days after conception?
A full-term pregnancy averages about 266 days (38 weeks) from conception, which equals roughly 280 days (40 weeks) from the last menstrual period. The calculator adds 266 days to the estimated ovulation date to produce the due date.
Does a longer menstrual cycle change the estimated conception date?
Yes. Because ovulation is estimated as cycle length minus 14 days, a longer cycle pushes ovulation and conception later, while a shorter cycle moves them earlier. This is why entering your actual average cycle length changes all the results.
Can you get pregnant from intercourse a few days before ovulation?
Yes. Sperm can survive in the reproductive tract for up to about five days, so intercourse in the days leading up to ovulation can result in conception on ovulation day. That is why the fertile window covers several days rather than one.
Will an ultrasound match this calculator's conception estimate?
Not always. An early ultrasound measures the embryo directly and may shift the estimated timeline if ovulation happened earlier or later than the cycle math assumes. Clinicians generally rely on the scan date when there is a meaningful difference.
Am I 4 weeks pregnant if I conceived 2 weeks ago?
Yes, in a typical 28-day cycle. Pregnancy is dated from the first day of your last menstrual period, which comes about two weeks before ovulation and conception, so conceiving two weeks ago makes you about four weeks pregnant by standard dating. With a longer cycle the gap is larger: in the 30-day example, ovulation is 16 days after the LMP, so two weeks after conception would be 4 weeks and 2 days.
How can I calculate my pregnancy from conception?
Count the days since conception and add 14 to get your gestational age in the standard LMP-based weeks, so 21 days since conception is 35 days, or 5 weeks, pregnant. For the due date, add 266 days (38 weeks) to the conception date: in the example, 2026-05-17 + 266 days = 2027-02-07. Because conception is itself an estimate, an early ultrasound may shift these dates by a few days.
How do I calculate my fertility date?
Your fertile days are the five days before ovulation through ovulation day, and ovulation is estimated as your last period start date plus (cycle length minus 14). For a period starting 2026-05-01 with a 30-day cycle, ovulation is May 1 + 16 = May 17 and the fertile window is May 12 to May 17. Enter your own LMP and average cycle length in the calculator to get the dates for your cycle.
What are the 5 most fertile days?
The five most fertile days are the four days leading up to ovulation plus ovulation day, with conception most likely from intercourse in the two days before ovulation and on the day itself. In the example (LMP 2026-05-01, 30-day cycle, ovulation May 17), that is May 13 through May 17. The calculator shows a six-day window starting a day earlier because sperm can survive up to five days.
Do you count the day you ovulate as day 1?
No. Cycle day 1 is the first day of your period, and in a 28-day cycle ovulation lands around cycle day 14. When people count days past ovulation (DPO), ovulation day is 0 DPO and the following day is 1 DPO. Pregnancy weeks are counted from the first day of the last period, not from ovulation.
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