This rule is used to estimate the date of confinement (estimated due date).
Estimated date of confinement = date of LMP - 3 months + 7 days + 1 year
Here, LMP = last menstrual period
Normal human pregnancy lasts 280 days (40 weeks) from LMP
Based on cycles lasting 28-30 days and regular
Contraceptives may change reliability of LMP
Bickley LS, Thompson, JE. The pregnant woman. In: Bickley LS, Szilagyi, PG. Bates’ Guide to Physical Examination and History Taking. 9th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2007: 817-838.
Bernstein HB, Weinstein M. Normal Pregnancy & Prenatal Care. In: Decherney AH, Goodwin TM, Nathan L, Laufer N, eds. CURRENT Diagnosis & Treatment Obstetrics & Gynecology. 10th ed. New York: McGraw-Hill; 2007:192.