Pregnancy Calculator
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.
Tip: The first trimester is the most critical for fetal development. Folic acid supplementation (400–800 mcg/day) before and during early pregnancy significantly reduces neural tube defect risk.
- 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
Ultrasound dating
An early ultrasound (8–14 weeks) gives the most accurate gestational age. If it differs from LMP by more than 7 days, the ultrasound date is used for clinical management.
Premature vs post-term
Births before 37 weeks are premature; after 42 weeks are post-term. Full term is 39–40 weeks. Gestational age at birth is one of the strongest predictors of neonatal outcomes.
| Trimester | Weeks | Key Development |
|---|---|---|
| First | 1–12 | Major organs form; miscarriage risk highest |
| Second | 13–26 | Movement felt; anatomy scan at week 20 |
| Third | 27–40 | Rapid growth; lung maturation; birth |
| Full term | 37–40 | Baby ready for birth |
| Post-term | 42+ | Induction often recommended |
Fun Fact
Only about 4% of babies are born exactly on their due date. Most births occur within 2 weeks before or after.