Pregnancy Due Date Calculator
Estimate your pregnancy due date based on your last menstrual period or conception date.
See your current week and trimester.
Pregnancy due date is calculated using Naegele’s Rule, the standard clinical formula developed by German obstetrician Franz Karl Naegele in 1812 and still used worldwide today.
Naegele’s Rule:
EDD = LMP + 280 days (40 weeks)
Where LMP is the first day of the Last Menstrual Period and EDD is the Estimated Due Date. This assumes a standard 28-day cycle with ovulation on Day 14.
A quick mental shortcut version: EDD = LMP + 9 months + 7 days
For irregular cycles, the formula is adjusted:
Adjusted EDD = Standard EDD + (Actual Cycle Length − 28 days)
Current Gestational Week is calculated as:
Gestational Age (weeks) = (Today’s Date − LMP) / 7
Worked Example: LMP = January 10, 2025:
- Add 9 months → October 10, 2025
- Add 7 days → October 17, 2025 (EDD)
- If today is April 1, 2025: (April 1 − Jan 10) = 81 days ÷ 7 = Week 11, Day 4
Trimester Breakdown:
- First trimester: Weeks 1–13 (organogenesis, highest miscarriage risk)
- Second trimester: Weeks 14–27 (fetal growth, anatomy scan at Week 20)
- Third trimester: Weeks 28–40 (lung maturation, positioning)
Note: Only 4–5% of babies are born on their exact due date. Most births occur within a ±2 week window around the EDD. Ultrasound measurements (crown-rump length before Week 12) are more accurate than LMP calculations when cycles are irregular.