How to Calculate Pregnancy
What is Pregnancy?
A pregnancy calculator estimates your due date, current gestational age, and trimester based on your last menstrual period (LMP) or conception date. Pregnancy lasts approximately 40 weeks (280 days) from the LMP, though normal term is considered 37–42 weeks.
Step-by-Step Guide
- 1Identify the first day of your last menstrual period (LMP)
- 2Add 280 days (40 weeks) to get the estimated due date (Naegele's Rule)
- 3Calculate gestational age: today − LMP in weeks and days
- 4Determine trimester: weeks 1–12 (first), 13–26 (second), 27–40 (third)
- 5If using conception date, subtract 14 days to estimate LMP
Worked Examples
Input
LMP: January 1
Result
Due date: October 8
Add 280 days to LMP
Input
Conception: January 15
Result
Due date: October 22
Conception = LMP + 14 days
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