Working back from the due date
A pregnancy is dated at about 40 weeks (280 days) from the first day of the last menstrual period, and conception usually happens around two weeks after that first day. So from a due date, the likely conception date is roughly 266 days earlier, and the last period about 280 days earlier. This tool does that arithmetic and shows a short window around the estimate.
Why it's a window, not a day
Conception can occur across the fertile window, and ovulation timing varies, so the exact day is rarely certain. Early ultrasound dating is generally the most accurate way to estimate these dates; this calculator is a convenient approximation from the due date alone.
Common uses
People use conception estimates out of curiosity, to align dates with events, or for record-keeping. Treat the result as approximate and confirm anything important with a healthcare provider.
Note: general information, not medical advice.
The trimesters of pregnancy
Pregnancy is divided into three trimesters, counted from the last menstrual period (LMP):
| Trimester | Weeks | Main milestone |
|---|---|---|
| First | 1–13 | Organs form |
| Second | 14–27 | First movements are felt |
| Third | 28–40 | Growth and preparation for birth |
The due date is estimated at 40 weeks from the LMP; only about 4% of babies are born exactly on that day.
Conception date vs. last period
Two references are often confused. The last menstrual period (LMP) starts the 40-week count, even though there is no pregnancy yet at that point. Conception happens at ovulation, about two weeks later, around 266 days before birth. That's why this tool subtracts 266 days from the due date to estimate conception, and 280 for the LMP (Naegele's rule).
Why it's a window, not an exact day
Fertilization can happen up to about five days after intercourse, because sperm survive that long waiting for the egg. The exact timing of ovulation also varies. That's why the result is shown as a window of several days rather than a single date; an early ultrasound refines the estimate.
Frequently asked questions
Is the conception date exact?
No, it's an estimate. Ovulation and fertilization timing vary, so treat it as a likely window rather than a precise day.
What if I only know my last period?
Conception is roughly two weeks after the first day of your last period on a typical cycle. A due-date or ovulation calculator can help from that starting point.
How is the conception date estimated?
It works back from the due date or last period. Conception usually happens about two weeks after the start of the last period, near ovulation.
How accurate is a conception date?
It's an estimate, not an exact day. Sperm can survive several days, so conception can fall within a window rather than a single date.
How is it related to the due date?
Pregnancy is counted as about 40 weeks from the last period, or roughly 38 weeks from conception. The tool uses that gap to place the likely date.
Is conception the day I had intercourse?
Not always. Fertilization can happen up to 5 days later, while sperm wait for ovulation.
Why is pregnancy 40 weeks if it's ~9 months?
Because it's counted from the last period, about 2 weeks before actual conception, which is roughly 38 weeks of effective gestation.
Can I know the exact conception date?
It's an estimate based on averages. A first-trimester ultrasound gives the most precise date.