Third Trimester Scans: Growth Checks and What They Measure

When and Why Growth Scans Are Ordered

For a lot of people, the 20-week anatomy scan is the last time they'll see their baby on a screen before the big day. Third trimester growth scans, usually done somewhere between 28 and 36 weeks, come into play when your provider wants a closer look at how your baby is growing. You might be scheduled for one if:

• Fundal height (the measurement from your pubic bone to the top of your uterus) is tracking above or below expected values
• A history of growth restriction or a large baby in a previous pregnancy
• Gestational diabetes or other medical conditions that can affect fetal growth
• Multiple pregnancy (twins or higher)
• Decreased fetal movement
• High or low amniotic fluid noted at an earlier scan

Some providers fold routine growth scans into their standard care plan for everyone, while others hold them for specific indications. Both approaches are reasonable.

What a Growth Scan Measures

A growth scan uses the same ultrasound technology you've seen before, but the focus shifts to a specific set of measurements that estimate your baby's size and check the surrounding environment.

Key Measurements

• BPD (Biparietal Diameter): The width of the baby's head.
• HC (Head Circumference): The distance around the baby's head.
• AC (Abdominal Circumference): The measurement around the baby's belly, which reflects liver size and nutritional status.
• FL (Femur Length): The length of the thigh bone.

Those four numbers feed into a formula that produces an estimated fetal weight (EFW). It's a mathematical model, not a direct measurement, and that distinction matters.

Why Estimated Fetal Weight Can Be Inaccurate

Here is one of the most important things to hold onto from any third trimester scan: the estimated weight is an estimate. Research consistently shows that ultrasound-based weight predictions carry a margin of error of roughly 10 to 20 percent in either direction. If your baby is estimated at 7 pounds, the actual birth weight could reasonably land anywhere from about 5 pounds 10 ounces to 8 pounds 6 ounces. Accuracy tightens in the middle and loosens at the extremes. Maternal body composition, fluid levels, and the baby's position all affect how well the measurements land.

Why does that matter? Because clinical decisions, including recommendations for early delivery or cesarean birth, can sometimes hinge on that estimated number. Knowing the margin of error helps you have a more informed conversation with your provider about what the figure actually means for your care.

The Biophysical Profile (BPP)

A biophysical profile pairs an ultrasound with (in the full version) a non-stress test to assess fetal well-being. The ultrasound portion looks at four components, each scored 0 or 2:

• Fetal breathing movements: The baby should practice breathing motions for at least 30 seconds during the observation window.
• Fetal movement: At least three discrete body or limb movements.
• Fetal tone: The baby should demonstrate at least one episode of flexion and extension (like opening and closing a hand or bending and straightening a limb).
• Amniotic fluid volume: At least one pocket of fluid measuring 2 cm or more.

A perfect score is 8 out of 8 on the ultrasound alone, or 10 out of 10 when the non-stress test is included. A score of 8 or above is reassuring. Lower scores may prompt additional monitoring, repeat testing, or, in some cases, a conversation about timing of delivery.

The BPP captures a specific 30-minute window. A lower score does not always indicate a problem. Babies sleep, and a sleepy baby may not perform all the required movements. Your provider will interpret the score alongside other clinical information.

The Non-Stress Test (NST)

The non-stress test monitors your baby's heart rate over 20 to 40 minutes using external monitors on your abdomen. It's called "non-stress" because nothing is done to stress the baby. Your provider is looking for:

• Accelerations: Temporary increases in heart rate that coincide with movement, a sign of healthy oxygenation.
• Baseline rate: Normal fetal heart rate at this stage is between 110 and 160 beats per minute.
• Variability: Healthy fluctuation in the heart rate, as opposed to a flat, unchanging line.

A "reactive" NST, meaning the baby showed expected accelerations, is reassuring. A "non-reactive" result may simply mean the baby was sleeping and is typically followed by a longer observation period or a BPP.

Amniotic Fluid Index

Amniotic fluid levels get checked frequently in the third trimester, either as part of a BPP or as a standalone assessment. Two common measurement methods are used:

• AFI (Amniotic Fluid Index): The deepest pocket of fluid is measured in each of four quadrants of the uterus and summed. A normal AFI is generally considered to be between 5 and 25 cm.
• Single Deepest Pocket (SDP): The deepest single vertical pocket of fluid. Normal is typically between 2 and 8 cm.

Low fluid (oligohydramnios) can raise concerns about placental function and may lead to increased monitoring or a conversation about delivery timing. High fluid (polyhydramnios) is often benign but can occasionally be associated with gestational diabetes or fetal conditions that affect swallowing.

What "Big Baby" and "Small Baby" Findings Really Mean

Few phrases in prenatal care carry as much weight, or cause as much unnecessary anxiety, as "your baby is measuring big" or "your baby is measuring small." Here is what those findings actually mean, and what they do not.

Large for Gestational Age (LGA)

A baby estimated above the 90th percentile is considered large for gestational age. Gestational diabetes is a common cause, but many babies who measure large are simply genetically big and are born vaginally without complications. ACOG (the American College of Obstetricians and Gynecologists) recommends against cesarean for suspected macrosomia unless the estimated weight exceeds 5,000 grams (about 11 pounds) in a person without diabetes.

Small for Gestational Age (SGA)

A baby estimated below the 10th percentile is considered small for gestational age. The critical distinction is between a baby who is constitutionally small (healthy, well-nourished, and simply on the petite side) and a baby experiencing intrauterine growth restriction (IUGR), where the placenta is not delivering adequate nutrition. Serial growth scans, Doppler studies, and clinical context help your provider make that distinction.

In both cases, a single measurement is never the whole story. Trends matter more than any individual data point.

Navigating Third Trimester Monitoring

If your provider recommends increased monitoring, these tools exist to provide information, not to signal that something is wrong. In the vast majority of cases, the result is reassuring. If you have questions about why a particular test is being recommended, ask. Understanding the reasoning helps you participate in shared decision-making about your care.

Find Your Provider Team on BAABY

Whether you need a maternal-fetal medicine specialist for high-risk monitoring or you want a midwife who takes the time to explain every result, BAABY's provider directory helps you search by specialty and location. Building a team that communicates openly makes every third trimester appointment feel more manageable.