A fetal growth scan is an ultrasound examination ordered to measure the size and estimated weight of a developing fetus, usually in the second or third trimester. The primary purpose is to monitor the baby’s growth velocity over time against standard growth curves for that stage of pregnancy. This assessment helps healthcare providers identify potential growth issues, such as Fetal Growth Restriction (FGR), where the baby is smaller than expected, or macrosomia, where the baby is significantly larger. Identifying these conditions allows for appropriate monitoring and management to optimize health outcomes for both the mother and the baby.
Differentiating Routine Anatomy Scans from Dedicated Growth Scans
The routine anatomy scan is typically performed between 18 and 22 weeks of gestation and includes initial measurements of fetal body parts. This scan is primarily a detailed check of the baby’s structural development and organ systems. While it provides baseline biometric data, it is not considered a dedicated growth scan for clinical surveillance.
A dedicated growth scan is performed later in the pregnancy, usually starting in the third trimester. It focuses on assessing the trajectory of growth rather than just the anatomy. This scan is often a repeat examination designed to measure growth velocity by comparing current measurements to previous ones. For low-risk pregnancies, a dedicated third-trimester scan is not routinely performed unless a specific medical reason arises.
Standard Monitoring Schedules for High-Risk Pregnancies
For high-risk patients, a scheduled series of growth scans provides ongoing surveillance of fetal well-being. Risks include chronic hypertension, pre-gestational or gestational diabetes, renal impairment, or a history of a previous baby being small for gestational age. Monitoring often begins in the late second trimester, typically between 26 and 28 weeks.
These serial scans are repeated regularly, most commonly every three to four weeks, to establish a growth pattern. A typical schedule might involve scans at 28, 32, and 36 weeks, continuing until delivery. The exact frequency and start time depend on the specific risk factor and the severity of the maternal condition. This sequential monitoring is the standard of care when the placenta’s ability to support growth may be compromised.
Key Clinical Indicators That Trigger Additional Scans
Even in low-risk pregnancies, certain clinical findings can prompt an unplanned referral for a growth scan. The most common indicator is an abnormal measurement of the symphysis-fundal height (SFH), which is the distance measured from the pubic bone to the top of the uterus. If the SFH is significantly smaller or larger than expected, or if the measurement shows static growth over several appointments, a growth scan is ordered to investigate the cause.
Sudden complications in the current pregnancy can also trigger an additional scan, regardless of previous risk status. Triggers include the diagnosis of new maternal conditions, such as preeclampsia, which affects placental function, or a significant decrease in the mother’s perception of fetal movement. Abnormal findings on other surveillance tests, like a non-stress test or a low result on a first-trimester blood screen for PAPP-A, can also mandate a referral for serial growth monitoring.
Data Collected During a Fetal Growth Scan
The core function of a growth scan is to gather specific biometric data to calculate the Estimated Fetal Weight (EFW). The sonographer measures four primary parameters: Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). The Abdominal Circumference is particularly important as it is a sensitive indicator of fetal nutritional status and growth.
These measurements are entered into a mathematical formula, such as the Hadlock formula, to generate the EFW, which is plotted onto a customized growth chart. Beyond size, the scan also assesses several well-being factors, including the volume of amniotic fluid, known as the Amniotic Fluid Index (AFI). A Doppler study is often performed to measure blood flow in the umbilical artery, providing information about placental nutrient and oxygen delivery.