How to Calculate Gestational Age

Gestational age is the length of time that a fetus has been developing in its mother's womb. This is different from how "far along" you are, because that term actually refers to the length of time that has passed since the first day of your last menstrual period. In actuality, your baby is usually conceived around day fourteen of your cycle. Here are a few ways to determine your baby's gestational age.

Instructions

Difficulty: Easy

1. Determine when the first day of your last menstrual period was and calculate how far along you are in your pregnancy. Count the number of weeks that have passed since the first day of your last menstrual period. If your last menstrual period started on September 5th, then you would be 5 weeks pregnant on October 10th. The first day of your last menstrual period is when your pregnancy technically starts. However, since your menstrual period did not end for at least 5 to 7 days after that, you could not have conceived yet.
2. Subtract 2 weeks from how far along you are in your pregnancy to determine the gestational age of your baby. Conception usually occurs 2 weeks after the first day of your last menstrual cycle; this is when your baby began its development. If you are 5 weeks pregnant, your baby's gestational age is 3 weeks. If you are 25 weeks pregnant, your baby's gestational age is 23 weeks.
3. Check your due date, as confirmed by ultrasound. Count back 38 weeks. Since some women may ovulate later or sooner than others, this is the most accurate way to calculate your baby's gestational age. Even though a pregnancy is 40 weeks, a baby's gestational age is 38 weeks on its due date. Therefore, counting back 38 weeks from your due date will give you a relatively accurate result. This will take a little more time than the first method.