Gestational Age Calculator
Calculate gestational age in weeks from last menstrual period.
Returns trimester, estimated due date, fetal milestones by week, and prenatal appointment dates.
Gestational age is the standard way of measuring pregnancy duration, counted from the first day of the last menstrual period (LMP) — not from conception. This convention means gestational age at conception is approximately 2 weeks (since ovulation typically occurs ~14 days after LMP).
Gestational age from LMP: Gestational Age (weeks) = (Today’s Date − LMP Date) ÷ 7
Due date (Naegele’s Rule): Due Date = LMP + 280 days (40 weeks)
Or equivalently: Due Date = LMP + 9 months + 7 days
Gestational age from ultrasound (crown-rump length in first trimester): Gestational Age (days) ≈ 42 + 1.75 × CRL (mm) (valid for CRL 10–84 mm, approximately 7–14 weeks)
Where:
- LMP: first day of last menstrual period (the standard reference point)
- 280 days: average human gestation (40 weeks from LMP)
- CRL: crown-rump length measured by ultrasound in millimeters
- Trimester breakdown: First trimester = weeks 1–13; Second = weeks 14–27; Third = weeks 28–40
Key gestational milestones:
- Week 4: Positive pregnancy test
- Week 6–8: Heartbeat detectable by ultrasound
- Week 10–12: First trimester screening (nuchal translucency)
- Week 20: Anatomy ultrasound scan
- Week 24: Viability threshold (can survive outside womb with intensive care)
- Week 37: Full term begins (previously 38–42 weeks was “term”)
- Week 40: Estimated due date
Worked example: LMP was February 10, 2026. Current date: April 8, 2026.
- Days elapsed: Feb 10 → Apr 8 = 18 days (Feb) + 31 days (Mar) + 8 days (Apr) = 57 days
- Gestational age: 57 ÷ 7 = 8 weeks and 1 day
- Due date: February 10 + 280 days = November 17, 2026
Ultrasound dating is generally more accurate than LMP dating if the two differ by more than 7 days — the ultrasound date takes precedence.