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Conception Calculator (by LMP)

Calculate conception date based on the First Day of Last Menstrual Period (LMP). Adjusts for cycle length variation to pinpoint ovulation.

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About

For women with regular cycles, the Last Menstrual Period (LMP) is the most common benchmark used by obstetricians. Conception typically does not occur during the period itself. It occurs during the ovulation phase which happens roughly 14 days before the next expected period. This tool calculates the conception date by projecting forward from the LMP and adjusting for your specific cycle length. If your cycle is 28 days, ovulation is on day 14. If your cycle is 35 days, ovulation is likely on day 21.

LMP calculator menstrual cycle ovulation timing date of conception pregnancy planning

Formulas

The algorithm assumes the luteal phase (time from ovulation to next period) is relatively constant at 14 days.

Dovulation = DLMP + (CycleLength − 14)

This date represents the peak fertility moment when the egg is released.

Reference Data

Cycle Length (Days)Ovulation Day (Post-LMP)Luteal Phase (Avg)
21 (Short)Day 714 Days
24Day 1014 Days
28 (Standard)Day 1414 Days
30Day 1614 Days
35Day 2114 Days
40Day 2614 Days
45 (Long)Day 3114 Days

Frequently Asked Questions

If your cycle varies significantly (e.g., 25 days one month, 35 the next), LMP calculations are less reliable. Ultrasound dating in the first trimester is the gold standard for irregular cycles.
The luteal phase is the second half of the menstrual cycle. It starts after ovulation and ends when the next period begins. While the follicular phase (first half) varies in length, the luteal phase is consistently around 14 days for most women.
Implantation bleeding can occur 6-12 days after conception. It is often mistaken for a light period. If this occurred, your actual LMP might be a month prior to what you think.