How this conception calculator works
Choose due date (subtracts 266 days for a fertilization midpoint, ±7 days for biological spread, then widens an intercourse window), LMP (assumes ovulation/fertilization most often between days 11–21 after the first day of the period, with a midpoint at day 14), or ultrasound plus gestational age from LMP (backs out to fertilization ~14 days after implied LMP). Results include a likely fertilization range and an intercourse window using sperm survival of up to about 5 days before ovulation and about 1 day after for egg viability. This is educational — not a substitute for your OB team.
When did I conceive?
Gestational age is usually counted from the first day of the last menstrual period, not from fertilization. Biologically, pregnancy as embryo development begins after ovulation and fertilization — often about two weeks after that LMP in a 28-day cycle, but ovulation moves with cycle length and stress, illness, or travel. Sperm can wait in the reproductive tract for several days; the egg accepts fertilization for roughly a day. So intercourse across a multi-day window can yield the same pregnancy — a single calendar day is rarely provable from math alone.
Three methods for estimating conception
Last menstrual period
For regular cycles, ovulation is often modeled around days 11–21 from LMP. Longer cycles usually shift ovulation later; shorter cycles shift it earlier. Irregular menses make LMP-based estimates unreliable compared with first-trimester ultrasound.
Due date
Working backward: estimated fertilization ≈ due date − 266 days (38 weeks). Because due dates are themselves uncertain, combine with the ranges shown in the calculator rather than treating one day as exact.
Ultrasound
Crown–rump length in the first trimester (often ~8–13 weeks) is a strong dating tool — commonly within roughly a week. Later ultrasounds are less precise for dating. If LMP and first-trimester ultrasound disagree by more than about a week, many practices adjust to ultrasound.
Fertilization vs. conception vs. implantation
Fertilization is fusion of sperm and egg in the fallopian tube, forming a zygote that divides as it travels toward the uterus. Implantation is attachment to the endometrium, often roughly 6–12 days after fertilization; hCG rises after implantation — what home tests detect. In strict medical language, conception is sometimes used to include implantation, so “pregnant” is not asserted until implantation succeeds. Many fertilized eggs never implant; this calculator’s headline dates refer to fertilization timing unless your clinician uses another convention.
The fertile window
The egg is usually fertilizable for about 12–24 hours after release. Sperm may survive several days in fertile cervical mucus. Together, that produces roughly a week-long window where intercourse can lead to pregnancy — often modeled as up to five days before ovulation through about a day after. That is why this tool outputs ranges, not a single magic date.
Limits of accuracy
Ovulation shifts, sperm survival varies, and due dates are estimates. For legal paternity or critical clinical timing, use appropriate testing and professional judgment — not an online calculator.
Early signs and testing
Implantation spotting, sustained basal body temperature, or mild symptoms may hint at pregnancy but overlap with PMS. Home tests are most reliable from about the first missed period onward using concentrated morning urine. Blood tests can detect lower hCG earlier when indicated.
For full due-date and week-by-week scheduling (including IVF), use our Pregnancy Calculator.