How to use the gestational age calculator to accurately calculate gestational age. Gestational age refers to the length of time from the first day of the last menstrual period to the current date. It is an important criterion for predicting the date of delivery and the physical development of the baby. How to use the gestational age calculator to accurately calculate the gestational age is a skill that mothers preparing for pregnancy and pregnant mothers must master. The following is a detailed introduction from four aspects. Step 1: Determine the first day of the last menstrual period. Determine the first day of the last menstrual period to calculate pregnancy.