Gestational Age & Conceptional Age

Gestational age is the number of weeks that have passed since the first day of a woman's last normal menstrual period. Thus, a full-term pregnancy wou

Gestational Age

You've probably heard people say that a pregnancy typically lasts about nine months (or 38 weeks) from the time of conception until a full-term infant is delivered. But in the U.S., healthcare professionals typically calculate the length of a pregnancy by gestational age. Gestational age is the number of weeks that have passed since the first day of a woman's last normal menstrual period. Thus, a full-term pregnancy would be 40 weeks long.

A trimester is approximately 13 weeks, or one third of a typical full-term pregnancy.

Conceptional Age

Under South Carolina's abortion law, the first trimester is defined by conceptional age. Gestational age is not the same thing as conceptional age. Conceptional age is how much time has passed since actual conception (fertilization). Conception cannot take place until you ovulate, and that typically happens about 14 days after the start of your monthly period.

So, conceptional age will always be about 14 days younger than gestational age. The average length of a full-term pregnancy is about 280 days, or 40 gestational weeks from the first day of the last period. The average length of a pregnancy from the time of conception is about 266 days, or 38 conceptional weeks from the day of conception.

How to Calculate Gestational Age

Step 1. Find the date of the first day of your last menstrual period on a calendar.

Step 2. Count the number of full weeks that have passed from the date in Step #1 to today's date. For example, if your last menstrual period started on July 1st and today's date is August 1st, the gestational age is four weeks.

Another Way to Know Gestational Age: Ultrasound

Your healthcare provider can also use ultrasound to figure out the estimated due date. Ultrasound is a technique used by healthcare professionals to create an image of internal body parts or to monitor a pregnancy. The image is created from high frequency sound waves. Ultrasound uses the size of the fetus to determine the gestational age (the time that has passed since the first day of your last period).