About this calculator
The estimated due date (EDD) is calculated from the first day of your last menstrual period (LMP) using Naegele’s rule: LMP + 280 days, adjusted for cycle length. This calculator gives the EDD plus how far along you are now, trimester, and days remaining.
Naegele’s rule and cycle adjustment
Standard Naegele assumes a 28-day cycle with ovulation on day 14. EDD = LMP + 280 days = LMP + 9 months + 7 days. For longer or shorter cycles, adjust: EDD = LMP + 280 + (cycle length − 28). A 32-day cycle adds 4 days; a 25-day cycle subtracts 3.
Trimesters
- Trimester 1: weeks 1–12. Highest miscarriage risk, organogenesis, morning sickness common.
- Trimester 2: weeks 13–26. Often the "easiest" trimester — energy returns, baby movements felt.
- Trimester 3: weeks 27–40+. Final growth, sleep difficulty, preparation for birth.
Accuracy of EDD
LMP-based EDD is ±2 weeks accurate for women with regular cycles. First-trimester ultrasound dating (crown-rump length, 8–13 weeks) is more accurate (±5 days) and is the gold standard when available. If ultrasound and LMP disagree by more than 7 days, providers typically use the ultrasound date.
Only ~4% of births happen on the EDD
Most births occur in a 5-week window centered around the EDD: 80% between weeks 37–42. "Full term" is 39–40 weeks; "early term" is 37–38; "late term" is 41; "post term" is 42+ (induction typically recommended). The EDD is a planning anchor, not a deadline.
What to focus on by trimester
T1: prenatal vitamins with folate (400–800 mcg), avoid alcohol/teratogens, establish prenatal care. T2: anatomy scan around 20 weeks, glucose tolerance test, decisions about birth plan. T3: birth class, hospital tour, postpartum planning, watch for preeclampsia signs.