A walkthrough, end to end.
- 1
Pick whether you're starting from a due date, LMP, or current gestational age.
- 2
Enter the date or weeks. The calculator works backwards to estimate when conception likely occurred.
- 3
Read the conception window — typically a 5–6 day fertile window ending around ovulation.
Conception date estimation
Conception happens at ovulation, typically ~14 days after LMP in a 28-day cycle (or due date − 266 days). Sperm survive up to 5 days, so the actual conception window is the 5–6 days ending at ovulation.
What you can do with this.
Working out conception from due date
If you only know your due date, subtract 266 days (38 weeks). The calculator does this automatically and gives you a fertile window around the date.
Conception from LMP for irregular cycles
Standard LMP estimates assume a 28-day cycle. If yours is 35 days, conception is closer to day 21 — adjust the cycle length input for a better estimate.
Conception from current gestational age
If your provider has dated you at e.g. '12 weeks' on a specific date, the calculator subtracts gestational age + 14 days from that point to estimate conception.
Conception from ultrasound dates
If a first-trimester ultrasound dated your pregnancy at 8w2d on a specific date, that's the most accurate gestational-age anchor. Work backwards: subtract 8 weeks 2 days, then another 14 days, to estimate conception.
IVF and conception dating
For IVF pregnancies, conception is the day of fertilization (typically the egg retrieval day). Embryo transfer happens 3–5 days later. Use those exact dates instead of LMP-based estimates — they're far more accurate.
Twin pregnancies and dating
Twin conception still happens within the same fertile window (whether identical or fraternal). Dating is the same as singleton pregnancy; only growth charts and weight-gain targets differ.
Conception calculator for late or missed periods
If your last period was unusual (light, short, mistimed), implantation bleeding may have been mistaken for it. An early ultrasound is the most accurate way to date the pregnancy when LMP is uncertain.
Why exact conception date is uncertain
Sperm survive 3–5 days inside the female reproductive tract, so intercourse anywhere in that window can result in conception. Even tracked ovulation has ±1–2 day variability. Treat the result as a window, not a date.
Pregnancy conception calculator 2026 — what's current
ACOG dating policy: first-trimester ultrasound (≤13w6d) takes precedence over LMP if discrepancy is >7 days. Beyond first trimester, ultrasound becomes less accurate; LMP and earliest scan are anchors.
Frequently asked.
It's a window, not a single day. Sperm can fertilize an egg up to 5 days after intercourse, and ovulation timing varies. Even an early ultrasound only narrows it to within a few days.
Conception calculators can suggest a likely window, but they're not reliable for paternity determination. DNA testing is the only definitive method.
In a typical 28-day cycle, ovulation happens around day 14. Pregnancy is dated from LMP (a measurable date) rather than ovulation (often unknown), so 'gestational age' includes 2 weeks before conception.
Yes — implantation bleeding (~10 days after ovulation) is usually lighter and shorter than a normal period. If your 'last period' was unusually light, the LMP-based dating may be off.
For intrauterine insemination, conception happens within 24 hours of the procedure. Use the IUI date as the conception date directly — it's more accurate than LMP-based estimation.
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