Pregnancy Calculator
28 days
Due Date--
Current Week--
Trimester--
From the moment a pregnancy is confirmed, two numbers matter most: how far along you are and when the baby is due. This pregnancy calculator works both out from the first day of your last menstrual period, counting gestational age in completed weeks and projecting a due date 280 days out, adjusted for cycles that run longer or shorter than 28 days. It also tells you which trimester you are currently in, so you can follow the pregnancy week by week.
Formula
dueDate = lastPeriod + 280 + (cycleLength − 28)
- lastPeriod
- First day of your last menstrual period
- 280
- Standard gestation of 40 weeks measured from the last period
- cycleLength − 28
- Adjustment for cycles longer or shorter than the 28-day average
How it works
- Enter the first day of your last menstrual period and your average cycle length.
- The due date is your last period plus 280 days (40 weeks), shifted by the difference between your cycle length and the standard 28 days.
- Current gestational age is the whole number of weeks since your last period, and the trimester is read from that week: 1 through 12 is the first trimester, 13 through 26 the second, and 27 onward the third.
Worked example
A last period began March 1, 2026, with a 28-day cycle.
- Cycle adjustment: 28 − 28 = 0 days.
- Due date: March 1 + 280 days = December 6, 2026.
- If the cycle were 32 days instead, the due date would shift four days later to December 10, 2026.
Estimated due date of December 6, 2026, with gestational age counted in weeks from March 1.
Frequently asked questions
- Why is pregnancy dated from the last menstrual period?
- Conception is hard to pinpoint, but the first day of the last period is a date most people know. By convention, the 40-week (280-day) count starts there, which means weeks one and two technically precede conception.
- How are trimesters divided?
- This calculator places the first trimester through week 12, the second from week 13 to week 26, and the third from week 27 to birth. Different sources draw the boundaries slightly differently, so a week near a cutoff may be labelled either way.
- How accurate is the estimated due date?
- Only about one in twenty babies arrives exactly on the estimated due date; most are born within two weeks either side. An early ultrasound often refines the date, especially for irregular cycles.
- What if my cycle is not 28 days?
- The calculator adjusts the due date by the difference between your cycle length and 28 days, since people with longer cycles tend to ovulate and conceive later. Entering your true cycle length improves the estimate.