How to use the conception calculator
- Choose what you know: the due date, the baby's birth date, or the first day of the last period.
- Enter the date. For a birth date, add how many weeks pregnant you were at birth (40 weeks if born on the due date); for a last period, your usual cycle length.
- Press Calculate for the likely conception date, the fertile days around it, and the matching due date.
How conception is dated
Doctors count pregnancy from the first day of the last period (LMP). Ovulation – and so conception – usually comes about two weeks later, and birth about 38 weeks after that:
\text{conception} \approx \text{LMP} + 14 \text{ days} = \text{due date} - 266 \text{ days}| You know | Likely conception |
|---|---|
| Due date | due date − 266 days (38 weeks) |
| Birth date and weeks at birth | birth date − weeks at birth + 14 days |
| First day of last period | LMP + 14 days, plus (cycle length − 28) |
March 15, 2027 − 266 days = Monday, June 22, 2026. Intercourse that led to the pregnancy most likely happened between June 17, 2026 and June 23, 2026.
How exact is it?
The date is an estimate, usually within a few days either side:
- Sperm survive up to five days, so sex several days before ovulation can lead to pregnancy.
- Ovulation varies. Even in regular cycles it can move by several days, and cycles longer or shorter than 28 days move it too.
- Due dates are estimates. Only about 1 in 20 babies is born on the due date, so working back from a birth date is less precise than from an early-scan due date.
A due date from a first-trimester ultrasound gives the most reliable conception date. For dating a pregnancy forward, use the due date calculator.
Can it prove paternity?
No. Because conception can follow intercourse by up to five days, and due dates themselves are estimates, a conception date can't reliably tell apart two possible dates of intercourse that are close together. Only a DNA test can establish paternity.
Frequently asked questions
How do I calculate when I conceived?
Subtract 266 days (38 weeks) from your due date, or add about 14 days to the first day of your last period.
How accurate is a conception date?
Usually within a few days. Ovulation timing varies and sperm can survive up to five days, so the exact day is uncertain unless the pregnancy came from IVF.
Which week of pregnancy is conception?
The start of week 3. Pregnancy is counted from the last period, so the first two weeks happen before conception.
Can I work out conception from my baby's birthday?
Yes – enter the birth date and how many weeks pregnant you were at birth. Without the length of the pregnancy, it is only a rough estimate.
How soon after conception can I take a pregnancy test?
Most home tests are reliable from the day your period is due – about two weeks after conception.