Gestation is a term used to describe the period of time between conception and birth during which a baby is developing in the uterus of the pregnant parent. The gestational age of a pregnancy refers to how far along the pregnancy is, and it's usually expressed as a combination of weeks and days.

Gestational age is used to estimate a possible due date, guide obstetric care and testing, and assess the baby's health at birth. It is used as a guide to determine if the baby is growing as expected and when to have certain prenatal examinations.

Let's dive deeper into the world of gestational age! We'll explore why this concept is so important, how it's calculated, and so much more in this informative guide.

How is Gestational Age Calculated?

It's not always easy for parents to know exactly when conception occurred. Your healthcare provider can figure out how many weeks you're pregnant by checking your menstrual cycle and using ultrasounds.

Using the menstrual cycle. The gestational age is calculated from the first day of the last menstrual period (LMP) of the pregnant woman to the present day. This is the most common way healthcare providers can calculate your pregnancy due date by counting 40 weeks from the first day of your last menstrual period (LMP). (Technically, the gestational age also includes the two weeks prior to conception, before the person becomes pregnant).

However, not all pregnancies last the full 40 weeks, and some may even go beyond this timeline. For this reason, this is an estimate, not a fixed date.

The due date can be estimated by adding 280 days (9 months and 7 days) to the first day of your last menstrual period (LMP).

Using ultrasounds. Calendar calculations that are based on the last menstrual period assume that the pregnant woman has a regular 28-day menstrual cycle. This is often not the case. Therefore, fetal ultrasounds provide more information to gauge gestational age. This ultrasound is used to calculate the estimated gestational age and estimated due date (EDD) of the fetus.

In most cases, doctors compare the EDD determined by ultrasound to the EDD determined by the last menstrual period.

After comparing these two numbers, your healthcare provider may decide to update your baby's due date if they think it's necessary.

Fertility Treatments Like IVF. If you've had fertility treatments such as in vitro fertilization (IVF), the due date calculations work differently. In this case, the reproductive endocrinologist will determine the due date on the basis of when the IVF process took place. This means that they will use the date of the embryo transfer (when the fertilized embryo is placed in the uterine cavity) to determine your due date.

Difference Between Gestational Age and Fetal Age

It's important to distinguish between gestational age and fetal age. Fetal age is calculated from the date of conception, while gestational age is calculated from the first day of your last menstrual period. This is during ovulation which means that the fetal age is about two weeks behind the gestational age.

This is the actual age of the fetus. However, it's a less accurate way to measure pregnancy because in most cases it's impossible to know when conception actually occurs in humans.

The Importance of Gestational Age

Gestational age is important because it guides prenatal care. It's an essential starting point for healthcare providers to:

Evaluate fetal growth
Find out the due date of the baby
Schedule and interpret prenatal tests and examinations
Assess risk status during pregnancy

Growth Stages Month by Month in Pregnancy

The fetus undergoes many changes during a typical pregnancy. This time is divided into three stages called trimesters. Each trimester lasts about three months. Your healthcare provider will probably talk to you about your baby's development in terms of weeks. So, if you're three months pregnant, you're about 12 weeks along.

During each trimester of pregnancy, you'll see distinct changes in the fetus and in yourself.

First Trimester: from conception to 12 weeks.

The first trimester is from conception to 12 weeks. This is usually the first three months of pregnancy. This is when the fertilized egg grows from a small group of cells to become a fetus that begins to take on human characteristics. The first trimester is exciting, but it is also when most people develop uncomfortable symptoms such as morning sickness and tiredness.

After 12 weeks (the end of the first trimester), your risk of miscarriage drops significantly because the most critical development has taken place. This is also when most people begin to feel some relief from their morning sickness.

By the time three months have passed, your little fetus is already about 2.5 to 3 inches long! That's about the size of a plum.

Second Trimester: from week 13 to week 28

Often considered the best part of pregnancy, the second trimester is the most comfortable. By this time, any morning sickness is likely to be over and the discomfort of the early stages of pregnancy has faded.

You may also begin to feel movement as the fetus flips and turns inside your uterus. It is during this trimester that many people find out the sex of the fetus. This is usually done during an anatomy scan (an ultrasound scan that looks at physical development) at about 20 weeks.

Third Trimester: from week 29 to week 40

This is the final stage of your pregnancy. It's tempting to count down the days until you're due, hoping for an early delivery, but each week of this final stage of development helps the fetus prepare for delivery.

Throughout the third trimester, the fetus gains weight quickly, adding body fat that will help after birth. The fetus is about 45 to 50 centimeters long and weighs about 3 to 4 kilograms.