Conception Calculator
Estimate when you likely conceived, and your fertile window, from your due date or last period.
An estimate only. It can't pinpoint the exact day of conception or be used to establish paternity.
How to use the conception calculator
This calculator estimates when you most likely conceived, along with your fertile window and estimated due date. You can work it out from two starting points:
- Due date: if your doctor or midwife has given you an estimated due date.
- Last period: the first day of your last menstrual period, plus your average cycle length.
The result shows the most likely conception date, a likely range around it, your fertile window, and how many weeks pregnant you are today.
How the conception date is estimated
Conception usually happens around ovulation, when an egg is released. In a typical 28-day cycle, ovulation happens about 14 days after the first day of your period. Pregnancy lasts about 266 days (38 weeks) from conception to birth.
Example: if your last period started on March 1 and your cycle is 28 days, ovulation and conception most likely happened around March 15. With a 32-day cycle, it would be around March 19.
The second half of the cycle, from ovulation to the next period (the luteal phase), is fairly consistent at around 14 days. That's why longer cycles usually mean later ovulation rather than a longer luteal phase.
Why conception is a range, not a single day
It's not possible to pinpoint the exact day of conception from dates alone, for a few reasons:
- Sperm can survive in the body for up to about 5 days.
- An egg lives for about 12 to 24 hours after ovulation.
- Ovulation varies from cycle to cycle, even for people with regular periods.
So intercourse up to about five days before ovulation can lead to pregnancy. That's why the calculator shows a fertile window (about five days before ovulation through one day after) and a likely conception range.
The fertile window
If you're trying to conceive, the most fertile days are the two or three days leading up to and including ovulation. Signs that ovulation is near can include:
- Clear, stretchy cervical mucus, similar to raw egg white
- A small rise in your resting body temperature after ovulation
- A positive result on an ovulation predictor kit
- Mild one-sided pelvic twinges for some people
Can it tell who the father is?
No. Because sperm can survive for several days and ovulation timing varies, a conception date estimate can't reliably show which act of intercourse led to a pregnancy. If paternity is in question, only a DNA test can give a reliable answer.
How accurate is it?
An early ultrasound, usually in the first trimester, gives the most accurate dating of a pregnancy. If your due date came from an early scan, the conception estimate based on it will usually be more accurate than one based on your last period, especially if your cycles are irregular.
For a week-by-week view of your pregnancy, try the pregnancy calculator, or see the due date calculator for the full-term window.
Frequently asked questions
How do I work out when I conceived?
Subtract 266 days from your due date, or add your cycle length minus 14 days to the first day of your last period. The calculator does this and shows a likely range around the date.
How accurate is a conception date calculator?
It gives a good estimate, usually within a few days. It can't be exact because sperm survive several days and ovulation timing varies from cycle to cycle.
Can a conception calculator prove paternity?
No. Conception dates are estimates and can't show which encounter led to pregnancy. Only a DNA test can establish paternity.
What is the fertile window?
It's the days in your cycle when pregnancy is possible, roughly the five days before ovulation, the day of ovulation, and about one day after.
What if my cycles are irregular?
Estimates from your last period are less reliable with irregular cycles. Use your due date from an early ultrasound if you have one, as it's usually more accurate.