How this calculator works
Enter the first day of your last menstrual period (LMP) and your average cycle length, and this calculator estimates your due date using Naegele's rule, then shows your current gestational age and trimester as of any chosen date (today by default).
Naegele's rule is a widely used dating convention, not a prediction of the actual birth date — only about 5% of babies arrive on their exact estimated due date, with most arriving within about two weeks of it either way. If you have had a dating ultrasound, that measurement is more accurate than a calculation from LMP alone and should be used instead.
Naegele's rule
Estimated due date = LMP + 280 days (40 weeks), for a standard 28-day cycleCycle-length adjustment: due date shifts by (cycle length − 28) daysGestational age = number of days since LMP, shown as weeks and daysThis calculator assumes conception occurred around 2 weeks after the start of the last period, the standard assumption behind Naegele's rule. Irregular cycles make LMP-based dating less reliable — an early ultrasound is the more accurate method in that case.
Worked example: LMP January 1, 28-day cycle
- Naegele's rule: January 1 + 280 days = October 8 of the same year.
- As of April 1 (day 90 of the pregnancy): 90 ÷ 7 = 12 weeks, remainder 6 days — 12 weeks 6 days pregnant.
- 12 weeks falls before week 14, so this is the first trimester.
- If the cycle were 35 days instead of 28, the due date shifts 7 days later, to October 15.
Frequently asked questions
How accurate is a due date calculated from the last period?
It is a starting estimate, typically accurate to within about 1-2 weeks for someone with regular cycles. A first-trimester ultrasound measurement is generally considered more accurate and takes precedence when the two disagree.
Why does cycle length matter?
Naegele's rule assumes ovulation around day 14 of a 28-day cycle. With a longer or shorter average cycle, ovulation (and so conception) likely happened later or earlier relative to the last period, shifting the estimate by the same number of days.
When do trimesters start and end?
This calculator uses the common convention: first trimester through 13 weeks 6 days, second trimester from 14 through 27 weeks 6 days, and third trimester from 28 weeks onward.
What if I don't know my exact last period date?
The estimate is only as accurate as the date you enter. If your cycles are irregular or you are unsure of the date, a dating ultrasound from a healthcare provider is the more reliable option.
How many weeks pregnant am I on my due date?
Exactly 40 weeks. Naegele's rule sets the due date at 280 days after the first day of the last menstrual period, and gestational age is measured as the number of days since LMP, so 280 days works out to exactly 40 weeks (280 ÷ 7 = 40).