What Does Gestational Age Mean?

One of the first numbers you’ll hear at your first prenatal appointment is your gestational age—and from that point on, it drives just about everything in your pregnancy care. It tells your provider when to order screenings, helps estimate your due date, and gives the medical team a framework for evaluating your newborn’s size and development at birth. Here’s what that number actually means.

What is gestational age?

Gestational age describes how far along a pregnancy is in terms of weeks and days.

Because the exact moment of conception is rarely known (in spontaneous pregnancies it can take a couple of days for the egg and sperm to team up after sex!), gestational age is counted from the first day of the pregnant person’s last menstrual period (LMP). That means the clock starts about two weeks before an egg is even fertilized, but this convention gives providers a consistent starting point across every pregnancy.

A singleton pregnancy lasts an average of 40 weeks—or 280 days—from the LMP to the estimated date of delivery. Gestational age is the yardstick your care team uses to time ultrasounds, lab work, and glucose screenings—and it determines whether a baby is considered preterm, full term, or post-term.

How do you calculate gestational age?

The most common method: Your provider counts forward from the first day of your LMP. If you have a regular 28-day cycle, ovulation likely occurred around day 14—so by the time a pregnancy test turns positive, you’re typically about four weeks along.

But cycles aren’t always textbook. That’s where ultrasound comes in. ACOG recommends a first-trimester ultrasound (up to 13-6/7 weeks) as the most accurate method for confirming gestational age. Your provider measures the embryo’s crown-rump length to pinpoint how far along you are, and if it doesn’t match the LMP date, your due date may be adjusted.

The exception is when pregnancy is achieved through in vitro fertilization (IVF) or another assisted reproductive technology (ART). Here, the date of conception is known exactly! ACOG recommends that the ART-derived gestational age should be used to calculate the estimated due date. Providers use the known age of the embryo and the date of transfer, along with ultrasound measurements, rather than the LMP.

What’s the difference between gestational age and fetal age?

These two terms come up a lot, but they measure different things. Gestational age starts from the first day of the LMP. Fetal age (also called conceptional age) starts at fertilization, which typically occurs about two weeks after the LMP—so fetal age runs roughly two weeks behind gestational age.

If your provider says you’re 12 weeks along (gestational age), your baby is closer to 10 weeks old developmentally. Providers use gestational age because it offers a standardized starting point—even though it slightly overstates the biological age of the fetus.

What is “gestational age at birth”?

Gestational age at birth is the gestational age recorded on the day your baby is delivered. It helps the medical team anticipate and manage potential health concerns.

These are the categories that are used:

• Preterm: before 37 0/7 weeks
• Early term: 37 0/7 through 38 6/7 weeks
• Full term: 39 0/7 through 40 6/7 weeks
• Late term: 41 0/7 through 41 6/7 weeks
• Post-term: 42 0/7 weeks and beyond

There are generally fewer risks when delivering between 39 and 41 weeks, which is why doctors recommend waiting until at least 39 weeks to deliver.

What does “small for gestational age mean”?

A baby is considered small for gestational age (SGA) when their birth weight falls below the 10th percentile for babies of the same gestational age—for a full-term newborn, that generally means under about 5.5 pounds.

Not all SGA babies are unhealthy—some literally got it from their mama (or papa!) and are just small because their parents are smaller in stature. However, SGA can sometimes signal fetal growth restriction (FGR), which may result from placental insufficiency, maternal high blood pressure, or infection. SGA babies who experienced growth restriction face higher risks of low blood sugar, temperature instability, and breathing problems. Your provider will monitor growth and determine whether additional care is needed.

What does “large for gestational age” mean?

A baby is considered large for gestational age (LGA) when their birth weight is above the 90th percentile—for a full-term baby, that typically means over about 8 pounds, 13 ounces.

Some babies are LGA because their parents are larger. But the most common medical cause is gestational diabetes—elevated blood sugar in the parent means extra glucose for the baby, more insulin production, and faster growth. LGA babies face a higher risk of birth injuries, respiratory distress, and low blood sugar after delivery. But going to your prenatal appointments—including glucose screening—helps catch and manage these risks early.

More on Pregnancy:

• Early Signs of Labor That Mean It's Almost Go Time
• Do You Need a Doula?
• What Is a Membrane Sweep?
• The Truth About Your Top Pregnancy Fears

Disclaimer: The information on our site is NOT medical advice for any specific person or condition. It is only meant as general information. If you have any medical questions and concerns about your child or yourself, please contact your health provider.