A fetal ultrasound uses high-frequency sound waves to create real-time images of a developing baby within the womb. The technique works by sending sound pulses into the body and recording the echoes that bounce back from different tissues and fluids. The primary purpose of the scan is to monitor the baby’s growth and development, confirm viability, and accurately determine the gestational age. The resulting images allow healthcare providers to check the position of the placenta, assess amniotic fluid, and observe the baby’s movement and anatomy.
Understanding the Image: Shades, Dimensions, and Orientation
Interpreting the ultrasound image relies on understanding echogenicity, which is the ability of tissue to reflect sound waves back to the transducer. The image is presented in shades of gray, where brightness corresponds to the density of the tissue being scanned. Structures that contain fluid and do not reflect sound, such as amniotic fluid or the fetal bladder, appear completely black and are termed anechoic.
Dense tissues, like bone, strongly reflect sound waves, causing them to appear bright white on the screen; these are called hyperechoic structures. Soft tissues, such as muscle or organs, reflect sound moderately and show up as various shades of gray, referred to as hypoechoic. While a standard scan is a two-dimensional (2D) cross-section, technologies can render three-dimensional (3D) static volume images or four-dimensional (4D) real-time motion images. The orientation is typically displayed so the top of the screen represents the area closest to the mother’s abdomen where the probe is pressed.
Interpreting Key Fetal Structures
Visual interpretation begins in the first trimester with the identification of the gestational sac, which appears as a dark, fluid-filled circle with a bright white rim confirming an intrauterine pregnancy. Within this sac, the yolk sac is the next structure to emerge, visible as a small, bright ring. Soon after, the fetal pole, the earliest visible form of the embryo, becomes apparent as a small, linear structure adjacent to the yolk sac, and its presence confirms heart activity.
In later scans, typically around the second trimester, the focus shifts to a detailed anatomical survey requiring specific cross-sections. The fetal head is examined in a transverse view to identify the midline falx, a bright line representing the separation of the two brain hemispheres. A primary assessment is the four-chamber view of the heart, which shows the two atria and two ventricles, allowing for the evaluation of heart size, axis, and rhythm. Other organs, such as the fluid-filled stomach bubble and bladder, confirm that the baby is swallowing and processing fluid, and the movement of the limbs is observed for proper development.
Deciphering the Standard Measurements
Fetal biometry involves a series of measurements used to assess growth and confirm gestational age. In the first trimester, the Crown-Rump Length (CRL), the length from the top of the head to the bottom of the torso, is the most accurate measurement for determining the Gestational Age (GA). After the first trimester, the baby’s position changes, and other parameters become more reliable for growth tracking.
Later measurements include:
- The Biparietal Diameter (BPD), which is the width of the head.
- The Head Circumference (HC), which assesses the overall size of the cranium.
- The Abdominal Circumference (AC), measured across the fetal abdomen, which is a sensitive indicator of growth and weight.
- The Femur Length (FL), which measures the longest bone in the body for skeletal development.
These biometric parameters are combined using formulas to calculate the Estimated Fetal Weight (EFW). The EFW is compared to standard growth curves to assign a percentile and refine the Estimated Due Date (EDD).
Safety and Limitations of Fetal Ultrasound
Fetal ultrasound is widely regarded as a safe, non-invasive procedure when performed for medical necessity by a trained professional. Since it utilizes sound waves rather than ionizing radiation, decades of use have shown no evidence of harm to the developing baby when exposure is kept within established guidelines. Experts recommend against unnecessary or prolonged exposure, such as non-medical keepsake scans, to adhere to the principle of using the lowest possible energy output for a diagnostic image.
The ultrasound is a screening tool and is not perfectly accurate, particularly in late pregnancy. The Estimated Fetal Weight (EFW) can have a margin of error that increases as the baby grows, sometimes being off by as much as 10 to 15 percent. While the anatomy scan can detect a high percentage of congenital conditions, the scan’s resolution and the baby’s position can sometimes obscure views. Therefore, a normal scan cannot definitively rule out all possible structural or chromosomal abnormalities.