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What is the Gestational Age?

A. Pasbjerg
Updated Mar 03, 2024
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The age of an unborn baby is known as the gestational age. This calculation is often difficult to make completely accurately, as the exact date and time when conception occurs is rarely known. Therefore, it is typically necessary to base the number, counted in weeks, on a definite date such as the first day of the mother's last menstrual period.

In most cases, calculation of the gestational age assumes that conception has occurred 14 days after the first day of the last period. Since conception normally can occur anywhere from 11 to 21 days later, the use of 14 days is only an average estimate, with the potential for several days error. The due date of the baby is then assumed to be 40 weeks from the first day of menstruation; however, only a small percentage of babies are actually born on their expected due date. Women who want to know the estimated gestational age of their fetus and the child's expected due date can check with their doctor for that information; there are also a variety of websites that provide tools to do the calculation for them.

There are certain ways to estimate the gestational age that can provide more accuracy than basing it off of the last menstrual period. Typically, women will have specific physical signs that they are ovulating, such as changes in cervical position and mucous; those who are aware of these changes and also know when they had sexual intercourse may have a much more specific knowledge of what day conception happened. Ultrasound, particularly early in the pregnancy, can reveal physical indicators of the fetus's age. In cases where fertility treatments such as in vitro fertilization are used, the date and even the time of conception are definitely known. Once the baby is born, doctors may be able to evaluate its age based on physical development.

Knowing the true gestational age of a baby becomes particularly important at the time of birth. An infant born prior to 37 weeks is considered premature; babies in this situation are often susceptible to a variety of medical issues, such as intraventricular hemorrhages or respiratory distress. Understanding where the newborn is in its development can help doctors understand what issues are most likely to occur, and to develop a treatment plan which will give the baby the best possible care. Observation of certain physical characteristics such as length, weight, and reflexes right after birth help doctors make this determination.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
A. Pasbjerg
By A. Pasbjerg
Andrea Pasbjerg, a The Health Board contributor, holds an MBA from West Chester University of Pennsylvania. Her business background helps her to create content that is both informative and practical, providing readers with valuable insights and strategies for success in the business world.
Discussion Comments
By BabaB — On Sep 13, 2011

It's amazing what progress doctors have made in keeping very early babies alive and bringing them to a healthy state.

I'm very happy that so many multiple births of three to six have had a happy outcome. I have heard of many of these children, who are born so tiny and immature, are given wonderful treatment and are able to develop into healthy, happy children. We owe a lot to medical science.

By live2shop — On Sep 13, 2011

For many mothers-to-be, their babies are born within close proximity to the expected due date, and at about 40 weeks.

But for some, it is crucial to know the baby gestational age of the unborn baby, so that treatment of problems at birth can be planned for.

When I had my children, there was no ultrasound equipment. It must have been very frustrating for doctor and parents of the baby not to know much about the gestational age except by number calculations and measurements.

By manykitties2 — On Sep 13, 2011

@Sara007 - You sounded a lot like me when I had my first child. I had all of the pregnancy books and wanted to know what was happening every step of the way. I made sure that I knew all about normal gestational age and what the weeks really meant when it came to my baby's development.

I had an ultrasound just to confirm the gestational age of my baby because I wasn't 100% sure when my last period had been and wanted to get as close to an accurate due date as possible.

Now, with my second baby on the way I am just taking things easy. The baby books have been put away and knowing about normal gestational age and all that is behind me.

By Sara007 — On Sep 12, 2011

When I had my first baby I tried to learn everything I could about giving birth and fetal gestational age. I felt that knowing things like gestational age measurements gave me a lot of information about the child growing inside me.

I was probably a pest to my doctor during my first few months of pregnancy as I was always asking if my baby was one of those that was small for its gestational age. I shouldn't have worried too much, my baby was perfectly normal and its gestational age and weight were right on the charts when my son was born.

By Kat919 — On Sep 11, 2011

I went into labor naturally, but I had to convince my doctor not to induce when I went "over forty weeks."

A friend of mine had had a scheduled induction for gestational diabetes. Based on her last menstrual period, they were at thirty-nine weeks gestational age. Only problem was, when that baby came out it weight all of five pounds. She heard them saying it looked more like a thirty-six weeker (i.e., a late preemie) and her baby had to spend twenty-four hours in the NICU (neonatal intensive care unit).

So when my doctor started talking about induction, I told him to cool his jets! First we did a nonstress test, which showed that everything was fine. (Basically they hook you up to a fetal monitor for twenty minutes to see if you're contracting and how the baby's heartrate responds to that and to his own movements.)

Then when I was over forty-one weeks, we did a BPP (biophysical profile), basically a fancy ultrasound. That also showed that baby was fine, had plenty of amniotic fluid, etc. Fortunately, I went into labor the next day! And my baby looked "just right" - not too early, not too late (which I'm told has its own set of problems).

By ElizaBennett — On Sep 11, 2011

In my first pregnancy, gestational age was critical because I had complete placenta previa and required a scheduled C-section. We weren't going to be waiting until my baby was ready to come out. The goal was to schedule the surgery for as close to my due date as possible, but before I would go into labor. If we scheduled too early, I could wind up with a sick baby - and the latest research shows noticeable problems for babies born at 37 and 38 weeks. 39 is much better.

The problem is that I had ovulated on day 21, so I wasn't really due until a full week after my last menstrual period date. I had kept charts showing my basal body temperature, but of course doctors don't always like to just trust their patients on things like that!

Fortunately, I had had an eight-week ultrasound that showed dates much closer to what I was calculating than to their beloved last menstrual period date. I couldn't convince them to use my ovulation date to calculate when we should have the surgery, but I was able to convince them to use the ultrasound date.

A. Pasbjerg
A. Pasbjerg
Andrea Pasbjerg, a The Health Board contributor, holds an MBA from West Chester University of Pennsylvania. Her business background helps her to create content that is both informative and practical, providing readers with valuable insights and strategies for success in the business world.
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