The Third Ultrasound: Fetal Growth and Wellbeing Scan (Third Trimester)

In a typical, low-risk pregnancy, the first two ultrasounds—the Dating Scan (first trimester) and the Anatomy Scan (mid-trimester)—provide the core diagnostic information. The **third ultrasound**, often referred to as the Growth Scan or Fetal Wellbeing Scan, is generally performed only if a specific clinical question arises. This scan is critical in the late stages of pregnancy (usually after 28 weeks) because its focus shifts entirely from fetal formation to assessing **fetal function** and the efficacy of the placenta.

As a specialist in maternal and child health, I explain that the third ultrasound is a powerful tool used to identify late-onset complications, manage high-risk conditions, and confirm readiness for delivery. It is an informed, targeted approach to ensure the safest delivery possible.

Table of Contents

1. Primary Goals of the Third Trimester Scan

The objective of this final scan is fundamentally different from the earlier checks. The baby's anatomy is already complete; the focus now is on how well the baby is thriving in the intrauterine environment.

Core Goals of the Growth Scan

• Growth Assessment: To ensure the baby is growing at the correct rate and that the estimated fetal weight (EFW) is within a healthy range.
• Wellbeing Check: To assess the health of the placenta and the oxygen and nutrient delivery systems (Doppler studies).
• Amniotic Fluid Volume: To measure the volume of amniotic fluid, a key indicator of fetal kidney function and hydration.
• Fetal Presentation: To confirm the baby's position (head-down, or cephalic; breech; or transverse) in preparation for labor planning.

2. Key Clinical Indications for the Third Ultrasound

A third-trimester scan is often triggered by clinical suspicion or known risk factors identified earlier in the pregnancy. If you receive an order for this scan, it is not a cause for alarm, but a necessary step to gather more data.

Common Reasons a Third Scan is Ordered

3. Essential Fetal Wellbeing Assessment Metrics

The Growth Scan involves two types of assessment: biometric measurement (size) and functional monitoring (health).

Biophysical Profile (BPP) Components

The BPP is a scorecard used to gauge fetal well-being, usually performed after 32 weeks. The scan assesses four parameters, with the fifth being the Non-Stress Test (NST), often performed externally:

1. Fetal Movement: Gross body movements (e.g., rolling, stretching).
2. Fetal Tone: Flexion and extension of a limb or hand (e.g., opening and closing a hand).
3. Fetal Breathing: Visible movement of the chest and diaphragm (practicing breathing).
4. Amniotic Fluid Volume (AFV): Measured either by the Amniotic Fluid Index (AFI) or the Single Deepest Pocket (SDP).

A high score (8/8 or 10/10 when including the NST) is highly reassuring, indicating the baby is robustly healthy and well-oxygenated.

4. Fetal Growth Assessment and Weight Estimation

The third ultrasound aims to determine the **Estimated Fetal Weight (EFW)**, which is crucial for identifying babies who are too small (Fetal Growth Restriction, FGR) or too large (Macrosomia, which can lead to shoulder dystocia at birth).

Accuracy and Error Margin

While EFW is useful, it is an estimate, not a precise weight. The margin of error is substantial, commonly around 10 to 15 percent in either direction. For instance, a baby estimated at 7 pounds could realistically weigh between 6 pounds and 8 pounds 2 ounces. Providers use the growth *trajectory* (comparing current size to earlier scans) more than the single final weight number.

Measurement Focus	Clinical Goal	Primary Concern if Abnormal
Abdominal Circumference (AC)	Most sensitive indicator of weight and nutritional status.	Fetal Growth Restriction (FGR) or being small for gestational age (SGA).
Amniotic Fluid Index (AFI)	Assesses fetal hydration and kidney function.	Oligohydramnios (low fluid) or Polyhydramnios (excess fluid).
Fetal Presentation	Confirms position for labor (Cephalic vs. Breech).	Breech position may necessitate external cephalic version or C-section planning.

5. Placental Function and Doppler Studies

One of the most valuable aspects of the late-trimester scan, particularly in high-risk pregnancies, is the **Doppler study** of blood flow.

Doppler Assessment of Placental Reserve

Doppler ultrasound measures the speed and direction of blood flow in key vessels, such as the umbilical artery and the Middle Cerebral Artery (MCA). This measurement is an indirect way to assess the health and "reserve" of the placenta.

• Umbilical Artery Flow: Measures blood resistance in the umbilical cord. High resistance suggests the placenta is struggling to transfer nutrients and oxygen effectively.
• Middle Cerebral Artery (MCA) Flow: Measures blood flow in the baby's brain. Changes here indicate the baby is redistributing blood to protect vital organs, a sign of distress known as the "brain-sparing effect."

Abnormal Doppler findings provide crucial, time-sensitive information that often dictates the timing and mode of delivery, sometimes requiring immediate intervention to safeguard the baby's health.

The third ultrasound is a targeted, functional analysis of the fetal environment. It is requested by your clinical team to answer specific questions about growth or well-being that cannot be answered by fundal height alone. By understanding the purpose of the BPP and Doppler studies, you gain confidence in the final, monitored phase of your pregnancy.