Pregnancy Conception Calculator
“When did I conceive?” Work backward from your due date, last period, or an ultrasound report to the most likely conception date, the realistic window around it, and the range of days when intercourse could have led to this pregnancy.
Conception vs. the pregnancy clock
Medicine counts pregnancy from the first day of the last menstrual period, but biologically nothing is conceived until ovulation — roughly two weeks later on a 28-day cycle. Working backward is therefore straightforward date arithmetic: a due date sits 280 days after the dating start and 266 days after likely conception. What limits precision is biology, not math. Ovulation drifts a few days cycle to cycle, sperm can wait in the reproductive tract for up to five days, and the egg itself is viable for less than a day. So the honest answer is always a date range: a tight one for fertilization and a wider one for the intercourse behind it.
How it’s calculated
Most likely conception = due date − 266 days. In last-period mode, conception = LMP + (cycle length − 14) days, since ovulation typically precedes the next period by ~14 days; in ultrasound mode the scan’s gestational age sets the dating start first. The likely window is ±3 days around the estimate (typical ovulation variability); the intercourse range extends 5 days before the window (sperm survival, per ACOG/ASRM’s six-day fertile window) and 1 day after it (egg viability of 12–24 hours).
Informational only, not medical advice — and never proof of paternity. Confirm dating with your provider, ideally against a first-trimester ultrasound.
Worked example
With a due date of February 14, 2027, the dating clock starts May 10, 2026, and the most likely conception date is May 24, 2026. The likely window is May 21–27, 2026, and intercourse anywhere from May 16 to May 28, 2026 could plausibly have led to the pregnancy.
Common mistakes
- Assuming conception happened on the day of intercourse — fertilization can follow sex by up to five days.
- Counting gestational age from conception; clinical weeks start ~2 weeks earlier, at the last period.
- Dating from a late ultrasound — scans before 14 weeks are far more accurate than third-trimester ones.
- Reading a ±3-day estimate as an exact day and building conclusions on it.
Where it is used
- Answering the classic “when did I actually get pregnant?” after a positive test.
- Cross-checking a clinic’s due date against what you know about your cycle.
- Reconstructing timelines around travel, medications, or exposures in early pregnancy.
- Context for paternity questions — with the firm caveat that only DNA testing can decide them.
Frequently asked questions
Why is conception about two weeks after my last period?
Pregnancy is dated from the first day of the last menstrual period, but the egg is not released (and cannot be fertilized) until ovulation — typically about 14 days before the next expected period. So on the clinical clock, conception happens near “2 weeks pregnant,” and a 40-week pregnancy involves roughly 38 weeks of an actual embryo.
How precise can a conception date be?
Rarely to the day. Ovulation timing varies by a few days even in regular cycles, sperm survive up to about 5 days inside the body, and the egg is fertilizable for only 12–24 hours. That is why this tool reports a most likely date plus a window, and a wider range for the intercourse that could have led to it.
Which input gives the best estimate?
A first-trimester ultrasound is the most accurate anchor for pregnancy dating (ACOG Committee Opinion 700), so use ultrasound mode if you have a scan report. A provider-issued due date is next best, since it usually already reflects a scan. Last-period dating is weakest when cycles are irregular.
Does my cycle length change the answer?
Yes. Ovulation tracks the next period, not the last one: it lands about 14 days before your next period starts. With a 34-day cycle you likely ovulated around day 20, not day 14, so the calculator shifts the conception window by (cycle length − 28) days in last-period mode.
Can this settle a paternity question?
No. Conception windows are estimates, and windows from intercourse with different partners a week or more apart can still overlap once sperm survival and ovulation variability are included. Only DNA testing can determine paternity — use these dates as context, not evidence.