Gestational Age Calculator | Weeks Pregnant Today

How this calculator works

This tool estimates gestational age (GA) today, days until the estimated due date (EDD), and a trimester label. It follows standard obstetric dating rules used in clinical practice.

Primary dating methods

• EDD known: If you already have an EDD (for example from a prior clinical visit), GA today is the number of days between today and the last menstrual period (LMP) equivalent implied by that EDD. By convention, EDD corresponds to 280 days after LMP (40 weeks). The calculator counts backward 280 days from the EDD to infer LMP, then forward to today.
• LMP + average cycle length: Naegele’s rule assumes a 28-day cycle with ovulation about day 14. If the reported average cycle differs, we adjust the assumed LMP-to-ovulation interval by the difference from 28 days (e.g., 31-day cycles → +3 days). EDD = LMP (adjusted) + 280 days.
• Ultrasound GA + scan date: If an ultrasound reported GA (weeks + days) on a specific scan date, the calculator adds the remaining days to 280 to obtain the EDD, then derives GA today by date differences. (Clinical practice generally trusts first-trimester ultrasound over LMP when they disagree meaningfully.)
• Conception/intercourse date: For approximate dating, LMP is estimated as 14 days before conception, then EDD = LMP + 280 days. This mirrors the obstetric convention that GA counts from LMP, not from fertilization.

What we show

• Gestational age today: weeks + days, computed from the inferred LMP (or directly from EDD) to the current date.
• Days to due date: calendar days between today and the EDD (never negative).
• Trimester label: first (0–13w 6d), second (14–27w 6d), third (28w+), following common clinical banding.

Assumptions & guardrails

• Dates are treated as whole days (local time), and outputs avoid negative values.
• Cycle-length adjustment is a practical heuristic. Irregular cycles, late ovulation, assisted reproduction, and uncertain dates may require clinician review.
• When both LMP and early ultrasound exist and disagree, early ultrasound dating is generally preferred.