Pregnancy Calculator
Start from the first day of your last menstrual period (LMP) or from a due date. From your last period you get the estimated due date and the estimated conception date. From a due date you get the estimated conception date and the matching first day of your last period. Once 7 days have passed since that first day, your current week of pregnancy and the trimester show as well. If the due date you entered has already passed, they do not. Every date is an estimate for a regular 28-day cycle.
How to use the Pregnancy 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.
- Two estimated dates appear. The current week and the trimester show below them once 7 days have passed since the first day of your last period, unless the due date you entered has already passed.
The due date calculation
Due date = LMP + 280 days (40 weeks) · · · Conception = LMP + 14 days · · · From a due date: LMP = due date − 280 days
The calculator counts from the first day of your last period. It assumes a 28-day cycle with ovulation on day 14 and has no field for cycle length. If you enter a due date, it first subtracts 280 days to find the matching first day. The current week is the number of days from that first day to today divided by 7, rounded down. Weeks 1 to 12 show as the 1st trimester, weeks 13 to 27 as the 2nd and week 28 onward as the 3rd.
Worked example
LMP: January 15, 2026. Estimated due date: October 22, 2026, 280 days later. Estimated conception: January 29, 2026, 14 days after the LMP. Checked on April 20, 2026, 95 days after the LMP, the calculator shows Week 13 and the 2nd trimester. Entering October 22, 2026 as the due date gives the same conception date, January 29, 2026, and January 15, 2026 as the first day of the last period.
Frequently asked questions
How accurate is the due date?
It is an estimate. The calculator adds 280 days, or 40 weeks, to the first day of your last period, so a period that started on January 15, 2026 gives a due date of October 22, 2026. It assumes a 28-day cycle and does not know the day you ovulated. The NHS says pregnancy normally lasts 37 to 42 weeks from that first day, and the 12-week scan estimates how far along you are more accurately. Your midwife or doctor sets the date your care is planned around.
What if my cycle is not 28 days?
The calculator has no cycle length field and always assumes 28 days. If you started from your last period, you can shift both dates yourself by the difference. With a 32-day cycle, that puts the estimated conception on day 18 and the due date 4 days later.
How do "weeks pregnant" get counted?
From the first day of your last period, not from conception. The calculator divides the days since that date by 7 and rounds down, so at Week 4 the estimated conception date is 2 weeks back. If you enter a due date, it counts from 280 days before that date. The count stops at Week 40.
Which weeks does each trimester cover?
In this calculator the 1st trimester runs through Week 12, the 2nd from Week 13 through Week 27 and the 3rd from Week 28 on. The Week and Trimester lines appear once 7 days have passed since the first day of your last period. If you start from a due date, the calculator puts that first day 280 days before it, and if the due date has already passed, neither line shows. Starting from your last period, Week 40 still shows after the due date.