The due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation at day 14. Only about 5% of babies are born on their exact due date — most deliver within 2 weeks before or after.

If your cycle is regularly longer or shorter than 28 days, your ovulation date differs from the assumed day 14. Adjustments can be made: add or subtract the difference from 28 days. Ultrasound dating in the first trimester is often more accurate for irregular cycles.

First trimester: weeks 1–12 (organ formation). Second trimester: weeks 13–27 (rapid growth, movements felt). Third trimester: weeks 28–40 (final development, preparation for birth). Each trimester has different milestones and prenatal care recommendations.

Naegele's rule is accurate to ±2 weeks for women with regular 28-day cycles. First-trimester ultrasound is the most accurate dating method (±5–7 days). Only about 5% of babies arrive on the exact due date. A 'term' pregnancy is 37–42 weeks.