Naegele's rule estimates the due date as the first day of the last menstrual period (LMP) plus 280 days — that is 40 weeks, or equivalently LMP − 3 months + 1 year + 7 days. It assumes a regular 28-day cycle with ovulation on day 14, so the estimate can shift for longer or shorter cycles. Gestational age is counted from the LMP: the number of completed weeks and extra days between the LMP and today.
Pregnancy is split into three trimesters: first (weeks 0–13), second (weeks 14–27) and third (week 28 to birth). This is an estimate only — about 1 in 20 babies arrives on the exact date, and your clinician may adjust it from an ultrasound. Everything is calculated locally in your browser using your device's time zone; your data is never uploaded.