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Why is the age of a fetus different from the length of pregnancy measured from the last menstrual period?

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Close-up of a home pregnancy test device showing a blue plus sign in the result window, indicating a positive result for pregnancy. — pregnancy Devices for individual use at home detect pregnancy by measuring levels of the hormone human chorionic gonadotropin (hCG) in urine.

The difference between the age of a fetus and the length of pregnancy measured from the last menstrual period (LMP) exists because the latter is a clinical convention used to estimate gestational age.

While the actual age of a fetus begins at ovulation and conception, the exact date of ovulation is often unknown. To provide a standardized starting point for medical care, physicians calculate gestational age by counting from the first day of the mother’s last menstrual period.12

Because this method assumes a standard cycle, it includes approximately two weeks of time before conception actually occurs. Consequently, the gestational age measured from the LMP is typically about two weeks longer than the true biological age of the fetus. Due to variations in ovulation timing, this calculation is an estimate, and clinicians may use ultrasound measurements of the fetus to more accurately determine developmental age.12