Conception Calculator
Enter the first day of your last menstrual period, or your due date, to estimate the day you conceived. The calculator places conception on day 14 of a regular 28-day cycle. Starting from your last period, it also shows your estimated due date. Starting from a due date, it shows the first day of your last period that matches it. Your current week of pregnancy and the trimester appear under the dates once 7 days have passed since that first day, unless the due date you entered has already passed.
How to use the Conception Calculator
- Under "Start from", pick "I know my last period" or "I know my due date".
- Enter that date in the date field and press the Calculate button.
- Starting from your last period, the result shows the estimated due date and, next to it, the estimated conception date. Starting from a due date, it shows the estimated conception date and the matching first day of your last period.
- The current week and the trimester appear below the dates once 7 days have passed since the first day of your last period. If you entered a due date that has already passed, they stay hidden.
How the conception date is estimated
Conception ≈ LMP + 14 days · · · Due date = LMP + 280 days · · · From a due date: conception ≈ due date − 266 days
The calculator assumes a regular 28-day cycle with ovulation on day 14, counted from the first day of your last period. It adds 14 days to that first day and shows the result as the conception date. A due date sits 280 days after the first day of the last period, so from a due date the calculator subtracts 280 days to find that first day and adds 14 back. That puts conception 266 days, or 38 weeks, before the due date. The cycle length is fixed at 28 days.
Worked example
LMP: May 4, 2026. Estimated conception: May 18, 2026, 14 days later. Estimated due date: February 8, 2027. Entering February 8, 2027 as the due date gives the same conception date, May 18, 2026, and May 4, 2026 as the matching first day of the last period.
Frequently asked questions
Is the conception date exact?
No. It is day 14 of a 28-day cycle, the day the calculator assumes you ovulated. In a 1995 study of 221 women trying to get pregnant, published in the New England Journal of Medicine, Wilcox and colleagues found that conception happened only when the couple had sex during the 6 days that ended on the estimated day of ovulation. So the sex that led to your pregnancy may date from up to 5 days before the date shown. If you ovulated earlier or later than day 14, the real date moves with it.
What if my cycle is not 28 days?
The calculator has no field for cycle length. If you start from your last period, move the conception date by the difference between your cycle and 28 days. That means 3 days later for a 31-day cycle and 2 days earlier for a 26-day cycle, on the same assumption that ovulation comes 14 days before the next period. If you start from a due date, the calculator counts back 266 days from it whatever your cycle length.
Can I use the due date from my scan?
Yes. Pick "I know my due date" and enter the date your midwife or doctor gave you. The calculator counts back 266 days, so a due date of February 8, 2027 gives an estimated conception date of May 18, 2026. It also shows May 4, 2026 as the matching first day of your last period. That is a counting date, and it can differ from the day your period really started.
Does this work after IVF?
Yes, through the due date. Cleveland Clinic dates an IVF pregnancy from the embryo transfer. Add 261 days to a transfer done 5 days after egg retrieval, or 263 days to one done 3 days after. Enter that due date here and the calculator counts back 266 days, which lands on the day of egg retrieval.
When do the week and trimester show?
Once 7 days have passed since the first day of your last period, or since the matching first day the calculator works out from a due date. The week is the number of days since that first day divided by 7, rounded down, and it stops at 40. Weeks 1 to 12 show as the 1st trimester, 13 to 27 as the 2nd and 28 onward as the 3rd. If you start from a due date that has already passed, neither line shows. If you start from your last period, Week 40 keeps showing after the due date.