Conception Date Estimate Calculator
Estimate how many days ago conception likely occurred from your last menstrual period, corrected for your cycle length and luteal phase rather than assuming day-14 ovulation.
Last updated: May 2026
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About this calculator
Conception happens at ovulation, so estimating the conception date means locating ovulation within the cycle and counting forward from the last menstrual period (LMP). The naive approach assumes ovulation on cycle day 14, but a more accurate estimate uses the fact that ovulation occurs on cycle day equal to cycle length minus luteal phase, because the luteal phase is the stable part of the cycle. This calculator takes the number of days since your LMP and subtracts that ovulation offset to return how many days ago conception most likely occurred — which is the same as the fetal (post-conception) age. Because gestational age is dated from LMP while conception age is dated from ovulation, the two differ by exactly the ovulation offset: about 14 days in a standard cycle, but more in a long cycle and less in a short one. The result is useful for understanding fetal development milestones, which are often described in post-conception weeks, and for sanity-checking a due date. All three inputs drive the answer: the elapsed-days input sets the baseline, while cycle length and luteal phase together fix how long after the period ovulation — and therefore conception — took place.
How to use
Worked example. It has been 56 days since the first day of your last period, your cycle averages 28 days, and your luteal phase is 14 days. Step 1 — find the ovulation offset from LMP: cycle length − luteal phase = 28 − 14 = 14 days. Step 2 — subtract that offset from elapsed days: 56 − 14 = 42 days since conception. That means the pregnancy is 8 weeks gestational age (56 days from LMP) but 6 weeks of fetal age (42 days from conception) — exactly the two-week difference clinicians expect for a textbook cycle. Now adjust for a longer cycle: with a 34-day cycle and a 14-day luteal phase, ovulation is on day 20, so conception was 56 − 20 = 36 days ago, making the fetal age younger even though the LMP date is unchanged. This shows why cycle length, not just the calendar, determines the conception estimate.
Frequently asked questions
How do I estimate my conception date from my last period?
Conception occurs at ovulation, which falls about cycle length minus luteal phase days after your last period began — not automatically day 14. So if it's been 56 days since your LMP and you ovulate on day 14, conception was about 42 days ago. Subtract the ovulation offset from the days elapsed since your LMP. Keep in mind it's an estimate: unless you tracked ovulation with tests or basal temperature, the exact day isn't known, and an early ultrasound gives the most reliable dating. This calculator does the cycle-adjusted arithmetic for you.
Is conception date the same as the date of intercourse?
Not necessarily. Sperm can survive in the reproductive tract for up to about five days, so fertilization can occur days after intercourse, whenever the egg is released at ovulation. That's why conception is dated to ovulation rather than to a specific act of intercourse. It also means the calendar date you conceived may not match a memorable date — the biologically meaningful moment is ovulation, which this calculator estimates from your cycle length and luteal phase. For legal or medical precision, clinicians rely on gestational age from ultrasound, not a presumed conception date.
Why does the conception estimate differ from my gestational age?
Because they use different starting points. Gestational age is measured from the first day of your last menstrual period, which is roughly two weeks before you actually conceived. Conception (fetal) age is measured from ovulation. The two always differ by your ovulation offset — cycle length minus luteal phase — which is about 14 days in a 28-day cycle but larger in longer cycles. So a pregnancy that is 8 weeks by gestational age is about 6 weeks by conception age. Developmental milestones are often quoted in post-conception weeks, which is why the distinction matters.