Conception — fertilization — happens within about a day after ovulation. The sex that led to it can be up to 5 days earlier, because sperm can wait for the egg, and ovulation itself is only estimated. Estimate both below: forward from your last period, or backward from your due date.
Because two different days are involved. Fertilization — conception — happens after ovulation, while the egg lives 12–24 hours. But the sex that led to it can be up to 5 days earlier, because sperm can wait for the egg. So sex on any day of a 6-day window ending on ovulation can lead to pregnancy:
Ovulation timing adds a second layer: luteal phases range 10–15 days in real-world data, so the ovulation day — and with it the conception estimate and the window’s end point — is itself ± a few days.
Whatever date you have — last period, due date or an early scan — converts to the same estimate. Here is one pregnancy worked through all three:
Last period Sep 1, 2026; regular 28-day cycle; due date June 8, 2027; a scan on Oct 20, 2026 measures 7 weeks 0 days.
Sep 1 + (28 − 14) = Sep 15 — estimated ovulation, with fertilization within about a day after it. Sex that could have led to it: Sep 10–15.
June 8, 2027 − 266 days = Sep 15, 2026.
Oct 20 − 7 weeks = Sep 1, the date the scan implies as “last period”; + 14 days = Sep 15.
If the routes give different days, the early scan wins (next sections), and your window moves with it.
With IVF, the egg is fertilized in the lab and a fresh embryo is transferred 2–5 days later; spare embryos can be frozen and transferred in a later cycle (HFEA). Pregnancy dating uses the transfer date and the embryo’s age, so the dates below are dating references:
| Embryo age at transfer | Dating reference (transfer − embryo age) | Due date | Equivalent last period |
|---|---|---|---|
| Day 5 (blastocyst) | Sep 26, 2026 transfer − 5 days | Jun 19, 2027 transfer + 261 | Sep 12, 2026 transfer − 19 |
| Day 3 | Sep 28, 2026 transfer − 3 days | Jun 21, 2027 transfer + 263 | Sep 14, 2026 transfer − 17 |
What these dates cannot tell you:
For your own transfer, the due date calculator’s IVF mode gives the due date, today’s week and the equivalent last period. Already have an IVF due date? Enter it above under “From due date” — the result is the same dating reference.
A longer cycle means later ovulation, so conception moves later by the same number of days. The days before ovulation stretch; the days after it barely change — the cycle length calculator shows this with data from 600,000+ cycles. For one last-period date:
| Your usual cycle | Ovulation ≈ conception | Sex that could lead to it | vs a 28-day cycle |
|---|---|---|---|
| 24 days | Sep 11 | Sep 6–11 | 4 days earlier |
| 26 days | Sep 13 | Sep 8–13 | 2 days earlier |
| 28 days | Sep 15 | Sep 10–15 | — |
| 30 days | Sep 17 | Sep 12–17 | 2 days later |
| 32 days | Sep 19 | Sep 14–19 | 4 days later |
| 35 days | Sep 22 | Sep 17–22 | 7 days later |
Clinicians count pregnancy from the last period, not from conception — so “weeks pregnant” runs about 2 weeks ahead of the time since conception:
Forward from here, the due date calculator gives the week and milestones; the full explanation is in why pregnancy is counted from the last period.
Last-period dating assumes ovulation on day 14 of a 28-day cycle. Long or irregular cycles break that assumption, so clinicians use a set rule to decide which date wins:
Compare the due date from the scan with the one from your last period.
From 9 weeks to 13 weeks 6 days the threshold is 7 days (ACOG Committee Opinion 700).
Your conception window stays where the calendar put it.
Scan adopted? Count back from the scan-based due date (− 266 days): that day is the new ovulation (and conception) estimate, and the 6-day fertile window ends on it — still ± a few days.
Only partly. Dates can make one partner less likely; they cannot confirm one. Three situations cover it:
The window is built on an estimated ovulation day that can be off by several days, so a date just outside it does not rule anyone out.
Any day in the 6-day window can lead to the pregnancy. A date inside it says “could be”, never “was”.
Sperm from either day can still be waiting at ovulation. Only a DNA paternity test can answer this.
Best estimate: around your ovulation day (last period + cycle length − 14) — fertilization happens within about a day after it. The sex that led to it can be up to 5 days earlier, and ovulation is only an estimate, so read the date as ± a few days.
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).
Ovulation timing varies (10–15 day luteal range), and without a measured ovulation day the estimate carries that error. Separately, the sex that led to the pregnancy can be up to 5 days before ovulation, so even a known conception day doesn’t pin the intercourse day.
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 estimate moves to the ultrasound-implied ovulation day, and the fertile window (the 6 days of sex that could have led to it) ends on that day — still ± a few days.
No. Dates can make one partner less likely, but they can’t confirm anyone — and because the ovulation day is only estimated, a date just outside the window doesn’t firmly rule anyone out either. Pregnancy can follow sex on any day of a 6-day window ending on ovulation (Wilcox 1995), so two partners inside the same window can’t be separated by dates. Only a DNA paternity test can answer this.
Roughly: conception ≈ desired due date − 266 days (the reverse of LMP + 280). The realistic target is the 6-day fertile window ending on 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.