Pregnancy Calculator
Track gestational age and trimester from your last period, conception date or due date, with key milestone dates.
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Fill in the fields and the answer updates instantly — no need to press anything.
How the pregnancy calculation works
Pregnancy is dated from the first day of your last menstrual period, not from conception, because that date is usually known. The 40-week figure is called gestational age; the baby has actually been developing for about 38 weeks.
If your cycle is longer or shorter than 28 days, ovulation shifts, so the calculation adjusts by the difference. A first-trimester ultrasound remains the most accurate way to date a pregnancy.
Each trimester lasts roughly 13 weeks: the first ends at 13 weeks 6 days, the second at 27 weeks, and the third runs to birth.
Worked example
Last period began 12 weeks ago, 28-day cycle.
- →Gestational age = 84 days = 12w 0d
- →Due date = last period + 280 days
- →Trimester: first (ends at 13w 6d)
Answer: 12 weeks 0 days, still in the first trimester, due in 196 days.
Pregnancy FAQs
Why does the due date assume 40 weeks?
Naegele's rule adds 280 days to the last period, based on a 28-day cycle with ovulation on day 14. It is a convention, not a precise prediction.
What if my scan gives a different date?
A dating scan in the first trimester is more accurate and is usually used as the official date. Use the known due date mode here to track from that.
How are trimesters counted?
First: weeks 1–13. Second: weeks 14–27. Third: week 28 to birth. Some clinicians differ by a week, but this is the most common split.
Pregnancy Calculator is part of the health calculators collection on Calcora, alongside 7 other tools. Results are calculated in your browser and are for information only — see our disclaimer.
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