Fetal ultrasound is a routine part of prenatal care, providing visual information about a developing baby inside the womb. It uses sound waves to create images, offering a safe and non-invasive way to observe the fetus. These scans play an important role in confirming pregnancy, estimating gestational age, and checking for multiple fetuses.
How Ultrasound Images Are Created
Ultrasound technology relies on high-frequency sound waves to generate images. A specialized device called a transducer emits these sound waves, which are beyond the range of human hearing. The transducer is placed on the pregnant person’s abdomen, often with a clear gel to ensure good contact. This gel helps the sound waves travel efficiently into the body and prevents air pockets from interfering with the transmission.
As these sound waves encounter different tissues and fluids, they bounce back as echoes. The transducer receives these echoes, which are converted into electrical signals. A computer processes these signals and translates them into a real-time image. The appearance of the image depends on how the sound waves are reflected.
Structures reflecting many sound waves, such as bone, appear bright white (hyperechoic). Tissues reflecting some sound waves, like muscle, appear in shades of gray. Fluids, such as amniotic fluid, reflect few or no sound waves and appear black (anechoic).
Understanding Fetal Measurements
Fetal ultrasound measurements assess gestational age and monitor growth. One of the earliest and most accurate measurements is the Crown-Rump Length (CRL), taken in the first trimester (7-13 weeks). This measurement, from the top of the baby’s head to its bottom, provides a precise estimation of gestational age. Establishing an accurate due date early in pregnancy is important for subsequent monitoring.
As pregnancy progresses, other measurements become relevant for assessing growth and development. The Biparietal Diameter (BPD) measures the distance across the baby’s head, between the two parietal bones of the skull. This measurement helps estimate gestational age and fetal weight, and monitors brain development. Head Circumference (HC) measures the perimeter of the baby’s head, offering another assessment of head size and brain development.
Abdominal Circumference (AC) measures the circumference around the baby’s belly, reflecting the growth of the liver and abdominal organs. This measurement monitors fetal weight and detects growth concerns. Femur Length (FL) measures the length of the thigh bone, the longest bone in the body. Femur length, along with BPD, HC, and AC, are combined to estimate the baby’s weight and assess overall growth. These measurements are compared against standard growth charts to ensure the baby is developing as expected.
Identifying Fetal Body Parts and Development
Ultrasound scans allow for the visualization of various fetal structures and developmental milestones. Early in gestation, the gestational sac and fetal pole (which develops into the embryo) are visible. The heartbeat can be detected as early as 6 weeks. As pregnancy advances, more detailed structures become visible.
During the second trimester (around 18 to 22 weeks), a comprehensive anatomy scan is performed. This scan examines the baby’s head, including brain structures like the cerebellum, and the skull. The spine is evaluated in multiple planes to check for proper formation. Major organs such as the four chambers of the heart, stomach, bladder, and kidneys are also assessed.
The development of limbs, including fingers and toes, can be observed. Fetal movement, such as stretches and kicks, becomes noticeable on ultrasound from around 8 to 10 weeks. By 12 weeks, swallowing amniotic fluid may be seen. The amount of amniotic fluid surrounding the baby is also assessed, as it plays a role in fetal development and well-being. These visual cues provide important information about the baby’s health and development.
What Else to Know About Fetal Ultrasounds
Fetal ultrasounds are safe for both the pregnant person and the baby, as they use sound waves instead of radiation. There are no known harmful side effects when used appropriately. While safe, certain factors can affect image quality, such as the baby’s position, the amount of amniotic fluid, or the pregnant person’s body habitus.
Fetal sex determination is a common question during ultrasounds. While not the primary medical purpose, the baby’s sex can be identified if the external genitalia are visible. This is possible after 13 weeks, with accuracy increasing significantly in the second trimester (18-22 weeks).
Different types of ultrasound scans serve specific purposes during pregnancy. A dating scan (first trimester) estimates the due date. An anatomy scan (second trimester) provides a detailed examination of the baby’s structures. Growth scans (later in pregnancy) monitor the baby’s size and well-being, assessing growth trends and amniotic fluid levels. These scans contribute to a comprehensive picture of the baby’s development.