Pregnancy Calculator
Find out how many weeks pregnant you are, which trimester you are in, your estimated due date and how much of the pregnancy is complete — all from your last menstrual period.
Last updated: June 20, 2026 · Medically reviewed by TG we-Calculate Editorial TeamFirst day of last period (LMP)
Today's date
12 weeks 5 days pregnant
- 1
Days elapsed since LMP
89Whole calendar days from the first day of the last period to today. - 2
Extra days within current week
89 − 12 × 7 = 5 - 3
Gestational age in weeks
⌊89 ÷ 7⌋ = 12
How is pregnancy dated from the last period?
Pregnancy is dated from the first day of your last menstrual period (LMP). Your gestational age in weeks is the days since the LMP divided by seven, and your estimated due date is the LMP plus 280 days (40 weeks). Conception happens about two weeks after the LMP.
Formula
How this is calculated
This calculator uses Naegele's rule and dates everything from the first day of your last menstrual period (LMP) — the input most people can recall — rather than from conception, which usually happens about two weeks later. Both dates are read as calendar days at midnight (no time component), so timezone and daylight-saving shifts do not move the result.
It counts the whole days elapsed between the LMP and the reference date, then divides by seven: the quotient is your completed gestational weeks and the remainder is the extra days (e.g. 90 days = 12 weeks 6 days). The trimester comes from that week — first is weeks 1–13, second weeks 14–27, third weeks 28 onward. The estimated due date is simply the LMP plus 280 days (40 weeks), built from the local date parts so it lands on the correct calendar day, and percent complete is the elapsed days divided by 280.
The maths assumes a regular ~28-day cycle with ovulation around day 14; for irregular or longer cycles, or after IVF, the LMP-based date can be off, and an early dating ultrasound is usually more accurate. Only about 1 in 20 babies arrives exactly on the due date, so treat these figures as a guide and confirm them with your midwife or doctor.
Examples
| Input | Result |
|---|---|
| LMP 2026-01-01, today 2026-04-01 | 12 weeks 6 days, 1st trimester, due 8 Oct 2026 (32.1% complete) |
| LMP 2026-01-01, today 2026-07-01 | 25 weeks 6 days, 2nd trimester, 64.6% complete |
| LMP 2026-01-01, today 2026-10-08 | 40 weeks 0 days, 3rd trimester, due date reached (100% complete) |
About this calculator
Pregnancy is conventionally dated from the first day of your last menstrual period (LMP), not from conception, because the LMP is a date most people can identify reliably. Counting from the LMP, a full-term pregnancy lasts about 280 days, or 40 weeks — this is Naegele's rule. Conception typically occurs around two weeks after the LMP, so "gestational age" runs roughly two weeks ahead of the actual age of the embryo.
The calculator divides the days elapsed since the LMP by seven to give your current age in completed weeks plus extra days (for example, "12 weeks 6 days"). It places you in the first trimester (weeks 1–13), second trimester (weeks 14–27) or third trimester (weeks 28–40+), estimates your due date as the LMP plus 280 days, and shows what fraction of the 280-day term has passed.
These dates are estimates. Only about 1 in 20 babies arrives on the predicted due date, and an early ultrasound is often more accurate than LMP dating, especially when cycles are irregular. Always confirm dates with your midwife or doctor.
Frequently asked questions
Count the days from the first day of your last menstrual period to today and divide by seven. For example, 90 days is 12 weeks and 6 days, since 12 × 7 = 84 with 6 days left over.
The 280-day (40-week) estimate is a useful guide, but only about 1 in 20 babies is born on the exact due date. An early dating ultrasound is often more accurate than the LMP, particularly with irregular cycles.
By convention, pregnancy is dated from the last menstrual period rather than conception, which usually occurs around ovulation about two weeks later. This is why gestational age runs roughly two weeks ahead of the embryo's true age.
Also known as
Sources & references
TG we-Calculate Editorial Team. (2026). Pregnancy Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/pregnancy-calculator
TG we-Calculate Editorial Team. "Pregnancy Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/pregnancy-calculator.
TG we-Calculate Editorial Team, "Pregnancy Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/pregnancy-calculator
@misc{wecalculate_pregnancy_calculator, title = {Pregnancy Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/pregnancy-calculator}}, year = {2026}, note = {TG we-Calculate} }
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