A sonogram, often called an ultrasound, is a non-invasive medical imaging technique that uses high-frequency sound waves to create real-time images of structures inside the body. It is commonly used during pregnancy to monitor a baby’s health and development. Sound waves are transmitted into the body, and their echoes are converted into a visual representation on a screen, offering a safe way to observe the developing fetus without using radiation.
Decoding the Sonogram Image
Sonogram images use shades of black, white, and gray to represent tissues and fluids. Fluid-filled areas, such as amniotic fluid or a full bladder, appear black (anechoic) because sound waves pass through them easily. Denser structures, like bones, reflect many sound waves and appear bright white (hyperechoic). Soft tissues, such as organs, appear in varying shades of gray; less dense tissue is darker, and denser tissue is lighter.
Image orientation provides spatial reference. The top of the image corresponds to the area closest to the transducer, the device placed on the skin, while the bottom of the image displays deeper structures within the body. The left side of the screen corresponds to the patient’s right side, and vice versa, though this can vary depending on the probe’s orientation. Sonogram images include labels and annotations like the date, patient ID, and estimated gestational age. These provide context for interpretation.
Identifying Key Fetal Structures and Milestones
In early pregnancy, around 4.3 to 5.0 weeks of gestation, the first identifiable sign on a sonogram is the gestational sac, appearing as a small, fluid-filled black circle within the uterus. By 5.1 to 6.0 weeks, the yolk sac, which provides early nourishment to the embryo, becomes visible inside the gestational sac as a small, round, echogenic ring. Soon after, the fetal pole, which will develop into the baby, can be observed as a focal thickening next to the yolk sac.
Around 6 to 6.5 weeks of gestation, fetal cardiac activity is detected, appearing as a flickering motion near the yolk sac. As pregnancy progresses, from 8 to 9 weeks, limb buds emerge, and the head becomes distinct from the body. By 10 weeks, the embryo is referred to as a fetus, and its eyes, ears, and neck begin to form, with fingers and toes becoming more defined.
During the second trimester, between 18 and 23 weeks, a detailed anatomy scan is performed to examine the baby’s developing organs and structures. This includes visualizing the fetal head, including brain structures such as the ventricles, choroid plexus, and cerebellum. The fetal face, including the profile, nasal bone, upper lip, and palate, is also examined for normal development.
Further into the second trimester, the sonographer checks the fetal heart for its rate, rhythm, and the presence of all four chambers and outflow tracts. The chest is examined for the diaphragm and lungs, while the abdomen is assessed for the stomach, liver, kidneys, bladder, and the umbilical cord insertion point. The spine is viewed in multiple planes, and the upper and lower limbs, including humeri, radius/ulna, femora, tibiae/fibulae, hands, and feet, are also inspected.
Understanding Common Sonogram Measurements
Sonogram measurements assess fetal growth, estimate gestational age, and monitor health. In early pregnancy, the Crown-Rump Length (CRL) is the most accurate measurement for dating, taken from the top of the embryo’s head to its bottom, between 7 and 13 weeks. This measurement is precise, within 3-4 days of the last menstrual period, and once established, the estimated due date remains unchanged.
Later in pregnancy, after 13 weeks, other measurements become relevant for tracking growth and development. The Biparietal Diameter (BPD) measures the distance across the baby’s head, from one parietal bone to the other, providing an assessment of head size. Head Circumference (HC) measures the circumference around the baby’s head, offering another indicator of head growth and brain development.
Abdominal Circumference (AC) measures the circumference of the fetal abdomen and is an indicator of fetal nutrition and overall growth, reflecting the development of organs like the liver and intestines. Femur Length (FL) measures the length of the femur, the longest bone in the body, which helps in assessing skeletal development and overall fetal length. These measurements, including BPD, HC, AC, and FL, are combined using formulas to estimate fetal weight (EFW).
When reviewing sonogram reports, you might encounter “percentiles” for these measurements. Percentiles compare a baby’s measurements to those of other fetuses at the same gestational age. For example, if a baby’s abdominal circumference is in the 75th percentile, it means it is larger than 75% of babies at that gestational age. Healthcare providers do not express concern unless a measurement falls below the 10th percentile or above the 90th percentile, as these ranges can indicate potential growth abnormalities.
Different Sonogram Types and Their Purpose
Standard pregnancy sonograms use 2D imaging, which produces flat, black-and-white cross-sectional views of internal structures. This type of ultrasound is widely used for routine checks, such as confirming pregnancy, monitoring fetal heartbeat and movement, and evaluating internal organs and overall development. It is the primary method for medical diagnostics due to its ability to clearly show anatomical details.
Beyond 2D, more advanced imaging options include 3D and 4D ultrasounds. A 3D sonogram creates a static, three-dimensional image by combining multiple 2D images from different angles. This allows for a clearer visualization of the baby’s external features, such as facial structure, and can be helpful in detecting certain physical abnormalities like a cleft lip or spinal issues.
Building on 3D technology, 4D sonograms add the element of time, providing live, moving 3D images. This dynamic view allows parents to see real-time movements, expressions, and behaviors of their baby inside the womb, offering a bonding experience. While 3D and 4D ultrasounds are popular for “keepsake” images, they are not used for primary medical diagnostics but can complement 2D scans in specific cases where more detailed visualization of external features is needed.
Doppler ultrasound is another specialized type that measures blood flow and pressure within blood vessels. It uses sound waves that bounce off moving blood cells, and the changes in frequency are converted into a visual representation of blood flow direction and speed. This type of sonogram is particularly useful for assessing blood flow in the umbilical cord or within fetal blood vessels, especially if there is a concern about the baby’s growth or conditions like Rh disease.