A prenatal ultrasound is a medical procedure using high-frequency sound waves to create images of the developing fetus and the mother’s reproductive organs. This imaging technique is a safe way to monitor fetal health, confirm viability, assess growth, and accurately determine the gestational age of the pregnancy. Understanding where the ultrasound probe is placed and why is helpful for anticipating what the experience will be like during the scan.
First Trimester Placement: Transvaginal Ultrasound
During the earliest weeks of pregnancy (typically before 8 to 12 weeks), the embryo and surrounding structures are small and deep within the pelvis. The transvaginal ultrasound (TVUS) approach uses a long, slender probe gently inserted into the vaginal canal to achieve the clearest possible image.
This internal placement positions the sound wave emitter much closer to the uterus, circumventing the need for waves to travel through the abdominal wall, which can blur the image. The high-frequency waves provide superior resolution for viewing small structures like the yolk sac, gestational sac, and the early fetal pole. Patients are usually asked to empty their bladder before this scan, ensuring the probe can be positioned optimally for clarity.
Standard Placement: The Transabdominal Approach
The transabdominal approach is the standard method used for the majority of the pregnancy, becoming the primary technique from the second trimester onward. This scan uses a larger, convex or curvilinear transducer designed to sweep across the surface of the abdomen. Water-soluble gel is applied directly to the skin to eliminate air pockets, which would otherwise scatter sound waves and prevent image formation.
In early pregnancy, the probe is often placed low on the abdomen, just above the pubic bone, to visualize the uterus deep in the pelvis. As the pregnancy advances, the uterus expands upward, requiring the sonographer to move the probe higher, often reaching the navel and the sides of the abdomen. The technician moves the probe in smooth, sweeping motions—longitudinal (up and down) and transverse (side to side)—to systematically view the entire volume of the uterus.
The probe’s movement allows for a comprehensive evaluation of the fetus, including its position, the location of the placenta, the amount of amniotic fluid, and necessary fetal measurements. For example, longitudinal sweeps over the fetal head measure the biparietal diameter and head circumference. Transverse sweeps help visualize structures like the four-chamber view of the heart, capturing multiple two-dimensional slices to gain a three-dimensional understanding of the anatomy.
Factors Influencing Probe Position
Probe placement is a dynamic process where the sonographer constantly adjusts the position to capture the clearest diagnostic images. A primary factor is the fetal position, or “lie,” since the baby’s orientation changes frequently, especially in the second trimester. If the fetus faces the mother’s spine, the sonographer must move the probe to the sides of the abdomen, as bone casts a shadow that blocks the view of underlying organs like the heart.
The location of the placenta also influences probe direction, particularly if it is situated on the anterior wall of the uterus, facing the mother’s abdomen. An anterior placenta can partially obscure the view of the fetus, requiring the technician to apply slight pressure or adjust the angle to see past the tissue. Furthermore, the necessary depth of penetration changes depending on the gestational age and the size of the mother’s abdomen.
Deeper structures, like the lower fetal body or the internal cervical opening, sometimes require the technician to press the probe more firmly into the abdomen to shorten the distance the sound waves must travel. Sonographers prioritize obtaining specific anatomical views, such as the fetal heart or brain structures, which dictates an immediate shift in probe placement to the most optimal acoustic window.
Practical Tips for Scan Day
Patient preparation directly affects the clarity of the ultrasound images and the ease of probe placement. For early transabdominal scans (up to approximately 16 weeks), a full bladder is often requested. The fluid acts as a natural window, pushing the uterus up and away from the pelvic bone and helping sound waves bypass gas in the bowel.
For routine scans after the first trimester (after 16 weeks), an empty bladder is usually preferred to maximize patient comfort during the longer exam and improve visualization of the lower uterine segment. Wearing comfortable, two-piece clothing is advisable, as it allows the abdomen to be easily exposed. Remaining calm and taking slow, steady breaths can help the sonographer by preventing tensing of the abdominal muscles, which can create resistance and make deep visualization more difficult.