Definitions of early pregnancy
Gestational age (also called menstrual age or menstrual dating) is estimated in completed days or weeks from the beginning of the woman's last menstrual period (LMP). Ovulation is assumed to occur two weeks after the LMP. Accurate menstrual dating depends upon knowledge of the LMP and a regular 28-day cycle length and is considered to have an accuracy of ±2 weeks. Embryonic or fetal age is calculated from the presumed date of ovulation and is therefore two weeks less than gestational age measured from the LMP.
The first trimester of pregnancy corresponds to the first 13 weeks' gestation and early pregnancy is typically defined 10 weeks’ gestation or less. The following table offers synonymous terms used to describe gestational age:
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