Due date calculated using Naegele's rule: LMP + 280 days (40 weeks). Formula assumes 28-day cycle with ovulation day 14. Actual delivery typically ±2 weeks from due date. Ultrasound in first trimester most accurate (±3 days).

Gestational age = weeks since LMP (Last Menstrual Period). Format: 'X+Y' (e.g., 20+3 = 20 weeks 3 days). Starts from LMP, not conception (conception is ~2 weeks after LMP). Standard pregnancy is 40 weeks (280 days).

Trimester 1 (0-13 weeks): Organs form, morning sickness common. Trimester 2 (14-26 weeks): Quickening (baby kicks felt), lowest miscarriage risk. Trimester 3 (27-40 weeks): Rapid growth, nesting instincts, Braxton-Hicks contractions.

Viability threshold: 24 weeks. Before 24 weeks: Baby unlikely to survive outside womb. 24-34 weeks: Survival possible with intensive medical care. 35+ weeks: Most organs mature; survival likely.

Yes. Naegele's rule assumes 28-day cycle. If your cycle is 30 days, ovulation occurs day 16 (not day 14), shifting due date ±2 days. Ultrasound dating more accurate than LMP if cycles are irregular.

Irregular periods make LMP dating less accurate. Solution: Ultrasound dating (most accurate in weeks 8-13, then less accurate). If unsure of LMP, get early ultrasound confirmation.

This calculator provides estimates using standard medical formulas (Naegele's rule). NOT a replacement for medical care. Always confirm with your doctor/OB-GYN at first appointment. This is for informational purposes only.

LMP (Last Menstrual Period) is when pregnancy dating starts (day 1 of pregnancy). Actual conception occurs ~14 days later (ovulation). So 'due date' = LMP + 266 days actual pregnancy, or LMP + 280 days gestational age.