The 32-week ultrasound is a common, non-invasive imaging procedure performed during the third trimester of pregnancy, typically between 32 and 36 weeks. Its primary purpose is to monitor the well-being of the fetus as the due date approaches. The scan provides a detailed look at how the baby is growing and assesses the environment inside the womb. This information helps healthcare providers confirm development is on track and identify potential issues requiring closer monitoring or intervention before delivery.
The Purpose and Logistics
The 32-week ultrasound, often called a growth scan, is typically ordered when there are concerns about the baby’s growth, if the pregnancy is high-risk, or if previous scans indicated a need for closer monitoring. While some providers perform this routinely, it is more common for conditions such as maternal hypertension, diabetes, or a history of complications in previous pregnancies to prompt this third-trimester check.
The procedure typically involves a transabdominal ultrasound, where a gel is applied to the abdomen and a transducer is moved across the skin. Unlike earlier scans, a full bladder is usually not necessary, but patients should follow specific preparation instructions. The entire appointment may take around 90 minutes, though the actual scanning time is often about 30 minutes, depending on the baby’s position and cooperation. The non-invasive nature of the scan makes it a safe way to assess the baby’s status.
Fetal Growth Assessment
The most significant component of the 32-week scan is fetal biometry, which involves taking specific measurements to estimate the baby’s size and growth trajectory. The sonographer measures four primary parameters: Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). The BPD measures the width of the head, and the HC measures the distance around the head, providing insight into head growth.
The Abdominal Circumference (AC) is particularly important as it reflects the baby’s nutritional status and fat stores. Femur Length (FL) measures the longest bone in the body, indicating skeletal growth. These individual measurements are then combined using established formulas, such as the Hadlock formula, to calculate the Estimated Fetal Weight (EFW).
The EFW is plotted on a growth chart to determine the baby’s percentile for the current gestational age. An EFW between the 10th and 90th percentiles is considered appropriate growth. A finding below the 10th percentile might suggest the baby is Small for Gestational Age (SGA), while a measurement above the 90th percentile may indicate the baby is Large for Gestational Age (LGA).
Evaluating the Support System
Beyond measuring the baby, the 32-week ultrasound provides a detailed check of the structures that sustain the pregnancy. A key assessment is the volume of amniotic fluid, measured using the Amniotic Fluid Index (AFI) or the Maximal Vertical Pocket (MVP) method. The AFI involves dividing the uterus into four quadrants and summing the deepest fluid pocket from each, with a normal range generally falling between 5 and 25 centimeters.
The amniotic fluid, primarily composed of fetal urine at this stage, provides a protective cushion necessary for fetal movement and lung development. The scan also evaluates the placenta’s location, ensuring it is not covering the cervix (placenta previa). The placenta’s appearance and grade are noted, which can indicate its maturity and function.
In certain pregnancies, the scan may include a Doppler flow study, which assesses blood flow through the umbilical cord and sometimes the fetal brain. This study helps ensure the baby is receiving sufficient oxygen and nutrients from the placenta. The sonographer also confirms the fetal presentation, noting the position of the baby in the womb, such as cephalic (head down) or breech, which is important for delivery planning.
What the Results Mean
A scan indicating normal growth, fluid levels, and positioning is reassuring, and monitoring continues with standard prenatal care. If the EFW is below the 10th percentile, Intrauterine Growth Restriction (IUGR) might be considered, requiring closer observation and additional Doppler studies. Low amniotic fluid, called oligohydramnios, is diagnosed with an AFI below 5 cm or a deepest pocket less than 2 cm.
Oligohydramnios can signal placental issues or other fetal concerns and may increase the risk of umbilical cord compression. Conversely, high fluid levels, known as polyhydramnios, may be associated with maternal diabetes or fetal swallowing difficulties. If the baby is in a non-cephalic position, such as a breech presentation, the provider will discuss options, including waiting for the baby to turn or planning an external version or Cesarean section. Any non-reassuring findings lead to consultation with an obstetric specialist and a plan for more frequent follow-up scans or non-stress tests.