Due date basics: what the calculator assumes
Pregnancy dating usually follows standard clinical conventions. This hub explains what “LMP” means, how conception-date estimates differ, and when cycle length matters.
LMP vs conception date (quick comparison)
| Method | Typical use | Simple rule of thumb |
|---|---|---|
| LMP | Most common clinical dating | EDD ≈ LMP + 40 weeks (280 days), adjusted for cycle length |
| Conception date | If you know ovulation/IVF timing | EDD ≈ conception + 38 weeks (266 days) |
Cycle length matters
Many rules assume a 28-day cycle. If your cycle is longer or shorter, ovulation timing may shift, which can shift the estimate.
Related pregnancy tools
Due date FAQ
Q:Is my due date exact?
No. It’s an estimate. Many pregnancies deliver before or after the due date, and early ultrasound can refine dating when LMP is uncertain.
Q:Why does gestational age start at LMP?
Clinically, pregnancy is commonly dated from the first day of the last menstrual period to standardize tracking, even though conception typically occurs about 2 weeks later in a 28‑day cycle.