How to Calculate Pregnancy Duration Based on the Date of Intercourse

In general, gestational age cannot be calculated based on the timing of sexual intercourse. Instead, pregnancy duration can be estimated using two primary methods:

1. For women with regular menstrual cycles, gestational age is typically calculated from the first day of the last menstrual period (LMP). Specifically, it is counted from Day 1 of the LMP up to the date pregnancy is confirmed. The number of days between these two dates represents the gestational age. For example, if the first day of the last menstrual period was July 5 and pregnancy was confirmed on July 25, the gestational age would be 20 days.

2. For women with irregular menstrual cycles—or those who cannot recall the date of their last menstrual period—gestational age may be estimated using ultrasound imaging or serum human chorionic gonadotropin (hCG) levels.

After becoming pregnant, many women mistakenly use the last day of their last menstrual period, the date of intercourse, or the date of ovulation as the starting point for calculating gestational age. However, this practice is inaccurate.