Conception clusters around ovulation — but sperm survive up to 5 days, so the honest answer is always a range, not a day. Estimate it two ways below: forward from your last period, or backward from your due date.
Two facts stack on top of each other. First, the egg is fertilizable for only 12–24 hours after ovulation — but sperm can survive up to 5 days waiting for it. So intercourse up to 5 days before ovulation can be the act that leads to conception, and the day you conceived is not necessarily the day you had sex. Second, ovulation timing itself varies: luteal phases range 10–15 days in real-world data. Combine the two, and any honest conception answer is a window of several days — the fertile window around your estimated ovulation day.
Clinicians count pregnancy from the last period, not from conception — so gestational age runs about 2 weeks ahead of the time since conception. When a result says "6 weeks pregnant," conception happened roughly 4 weeks ago. For the forward direction, the due date calculator shows gestational age and milestones; for what happens after conception, the implantation calculator covers the 6–12 day window when the embryo attaches.
LMP-based dating assumes ovulation on day 14 of a 28-day cycle. Real luteal phases range 10–15 days (npj 2019, 600,000+ cycles), so with long or irregular cycles the LMP-implied conception window can be off by days. When an early ultrasound and the LMP dates disagree by more than about 5–7 days, clinicians adopt the ultrasound (ACOG Committee Opinion No. 700).
What that means here: if you have a dating scan, anchor on it — your conception window is the fertile window around the ultrasound-implied ovulation day, still a range of several days, never a single day.
Best estimate: your ovulation day (last period + cycle length − 14). But conception can follow intercourse up to 5 days before ovulation — the honest answer is the fertile-window range, not a day.
Not necessarily — sperm survive up to 5 days, so the conception event (around ovulation) can stem from intercourse days earlier.
Conception ≈ due date − 266 days (the reverse of LMP + 280 with conception at day 14).
Two stacked uncertainties: ovulation timing varies (10–15 day luteal range), and sperm wait up to 5 days. Even perfect data yields a multi-day window.
Gestational age counts from the last period — about 2 weeks ahead of conception. "6 weeks pregnant" ≈ 4 weeks since conception.
To a range of several days, never a single day. Clinical dating uses first-trimester ultrasound (±5–7 days).
Only loosely. An estimated 14–25% of women have irregular cycles (NICHD), and for them the LMP-based estimate widens to a week or more — early ultrasound dating (ACOG Committee Opinion No. 700) is the meaningful answer, since cycle-based math can’t pin ovulation.
Fertilization happens within 12–24 hours of ovulation; implantation follows 6–12 days later.
Trust the early ultrasound. LMP dating assumes ovulation on day 14, but real luteal phases range 10–15 days (npj 2019, 600,000+ cycles), so long or irregular cycles throw LMP-based dates off. First-trimester ultrasound dating carries roughly a ±5–7 day margin and is what clinicians adopt when the two disagree (ACOG Committee Opinion No. 700). Your conception window is the fertile window around the ultrasound-implied ovulation day — still a range, never a single day.
Only to rule someone out, not to rule someone in. Conception lands in a 6-day fertile window (Wilcox 1995), and sperm survive up to 5 days, so intercourse on different days inside that window can all lead to the same pregnancy. If two partners fall inside the same window, no calculator can separate them — only a DNA paternity test can.
Roughly: conception ≈ desired due date − 266 days (the reverse of LMP + 280). The realistic target is the fertile window around that day — per-day conception probability in the Wilcox 1995 study population rose from about 10% five days out to about 33% on ovulation day. Treat any single target date as a hope, not a plan — only about 4–5% of babies arrive on their exact due date.