Pregnancy Conception Calculator

Understand conception timing and the fertile window

Why conception estimates vary

Conception typically occurs near ovulation, but exact timing differs among individuals and cycles. Sperm can survive in the reproductive tract for several days, and the precise ovulation day can shift with stress, sleep, travel, or illness. Conception timelines are best viewed as ranges, not single guaranteed dates.

Calculation methods

• LMP (Last menstrual period): Assumes ovulation about cycle length − 14 days after the first day of your period. We return an ovulation estimate and a 6‑day fertile window.
• EDD (Estimated due date): Counts backwards ~266 days to estimate conception and derives an approximate LMP and ovulation date.
• Ultrasound dating: Uses the ultrasound date and the gestational age reported to derive LMP and then estimate ovulation from your cycle length (default 28).
• Known ovulation date: Based on tracking (OPK, BBT, monitoring); conception is most likely the day before and the day of ovulation.
• Positive test date: As an educational approximation, conception is often 10–12 days before a positive pregnancy test; you can adjust this assumption.

Fertile window explained

The fertile window spans the days when intercourse can lead to conception—typically the five days before ovulation plus ovulation day. The probability is not equal across these days; it is usually higher in the two to three days immediately before ovulation, then drops rapidly after ovulation.

Cycle length and ovulation

People with longer cycles often ovulate later; shorter cycles often ovulate earlier. If your average cycle differs from 28 days, adjusting the ovulation estimate by cycle length − 14 improves accuracy. If your cycles vary a lot, use the method with the most reliable data you have (e.g., a known ovulation day or early ultrasound).

Dating by ultrasound or EDD

Early first‑trimester ultrasounds and a confirmed due date can anchor your conception estimate. We work backwards from EDD (~266 days from conception) or ultrasound weeks+days to derive likely conception and a corresponding fertile window.

Helpful tracking tips

• Track cycle dates: Record LMP and typical cycle length for a more tailored ovulation estimate.
• Use multiple signs: Combine OPKs, temperature shifts, and cervical mucus observations to pinpoint ovulation.
• Think in ranges: Focus on windows, not single dates, to reduce stress and improve planning.

Safety and limitations

This tool is for education only and is designed for global use. Conception timing can be influenced by many factors. For medical questions or pregnancy dating decisions, consult a qualified healthcare professional.

Frequently asked questions

• HOW TO estimate conception from my LMP? Enter your LMP and average cycle length. Ovulation ≈ cycle length − 14 days after LMP, and the fertile window is the five days before ovulation plus ovulation day.
• HOW TO find conception from my due date (EDD)? Count back about 266 days from your EDD to estimate the conception date. The calculator does this and shows an ovulation-based window.
• HOW TO use my ultrasound to estimate conception? Enter the scan date and gestational age (weeks+days). We derive an LMP, estimate ovulation from your cycle length, and show the fertile window and likely conception date.
• HOW TO narrow the window with OPKs/BBT? If you know ovulation day from OPKs/BBT, use the “Known ovulation” method; conception is most likely one day before to the day of ovulation.
• HOW TO estimate from a positive test? Conception often occurs ~10–12 days before a positive test, depending on test sensitivity. You can change this assumption in the calculator.
• HOW TO read the doughnut chart? The chart shows the relative likelihood across fertile window days, emphasizing the higher‑probability days just before ovulation.