Due Date Calculator

Due Date Calculator – Quick Cheat Sheet

Naegele’s rule: EDD = LMP + 280 days (40 weeks).
Cycle-length adjustment: If your cycle ≠ 28 days, add (cycle − 28) days to the EDD.
Gestational age: Days since LMP → shown as weeks + days (e.g., 18w 3d).
Trimesters: 1st (0–13w6d), 2nd (14–27w6d), 3rd (28w+).
Conception estimate: ~ LMP + (cycle − 14) days (ovulation approximation).
Inputs: LMP (YYYY-MM-DD), optional cycle length (20–45), optional “As of” date (defaults to today UTC).
Reality check: Ultrasound dating may refine EDD. This tool is an estimate, not medical advice.

Edge cases & notes

LMP after the “As of” date is treated as an input error to avoid negative gestational ages.
All calculations are calendar-accurate using PHP DateTimeImmutable in UTC.
Weeks/days until (or past) EDD use exact day counts (no average month assumptions).