An obstetric (OB) ultrasound, also known as a prenatal or fetal ultrasound, is a standard, non-invasive imaging procedure used throughout pregnancy. This technique employs high-frequency sound waves to generate real-time visual images of the developing fetus, the placenta, and maternal reproductive organs. A device called a transducer transmits sound waves into the body and captures the echoes that bounce back off internal structures. A computer translates these echoes into a two-dimensional image displayed on a screen, providing valuable information about the pregnancy.
Preparing for the Scan and What to Expect
Preparation for an OB ultrasound varies depending on the stage of pregnancy. For scans performed early in the first trimester, a full bladder is often requested because the fluid helps push the uterus into a better position for imaging. Later in the pregnancy, when the uterus is larger and positioned higher in the abdomen, a full bladder is usually not necessary. Patients are generally advised to wear comfortable, loose-fitting clothing.
Once in the examination room, the patient lies down on a table. A warm, hypoallergenic gel is applied to the abdomen; this gel eliminates air pockets between the skin and the transducer, ensuring efficient sound wave transmission. The sonographer moves the transducer across the gel-covered skin using moderate pressure to capture images from various angles. For very early or specific internal scans, a transvaginal approach may be used, which involves inserting a smaller, lubricated transducer into the vagina.
The entire procedure is typically painless, though pressure from the transducer over a full bladder or to reach a difficult angle may cause slight discomfort. After the necessary images are captured, the gel is wiped off. The images are then sent to a physician, such as a radiologist or obstetrician, for interpretation and a formal report.
Primary Diagnostic Functions
An OB ultrasound monitors the health of both the mother and the fetus. A primary application is confirming pregnancy viability by detecting a gestational sac and a fetal heartbeat, sometimes seen as early as six weeks. The scan is also the most accurate tool for determining gestational age, or “dating” the pregnancy, especially when performed in the first trimester. This is achieved by measuring the crown-rump length to estimate the expected delivery date.
A major component is the detailed anatomy scan, generally performed between 18 and 22 weeks, which systematically assesses the fetus from head to toe. During this comprehensive examination, the sonographer takes precise measurements of structures like the biparietal diameter (the width of the head), head circumference, abdominal circumference, and femur length to assess fetal growth. The internal organs, including the brain, heart, spine, and kidneys, are examined to screen for potential congenital anomalies.
The scan also provides information about the fetal environment, such as the location of the placenta, which is important for planning delivery. Assessing the volume of amniotic fluid surrounding the fetus is a routine part of the examination. Too much or too little fluid can indicate potential issues. Furthermore, the scan helps identify the number of fetuses in a multiple gestation pregnancy.
Understanding the Technology Variations
The standard diagnostic tool is the two-dimensional (2D) ultrasound, which produces flat, black-and-white cross-sectional images. This modality is highly effective for visualizing internal structures and is the foundation for all medical diagnostic measurements, such as those used in the anatomy scan. The 2D image is essentially a slice view, showing outlines and internal organs in detail.
Advancements led to three-dimensional (3D) and four-dimensional (4D) imaging, created by gathering multiple 2D images and using sophisticated software for rendering. A 3D ultrasound provides a static volume render, offering a clearer, lifelike view of the baby’s external features, such as the face and limbs. The distinction of the 4D ultrasound is the addition of time, which allows for the viewing of 3D images in real-time motion, essentially creating a video of the fetus.
Safety Profile and Typical Schedule
Ultrasound is a widely accepted procedure because it relies on sound waves rather than ionizing radiation, making it a safe imaging technique for both the expectant mother and the fetus. Decades of use in obstetrics have not resulted in any documented reports of adverse effects on the fetus from diagnostic procedures. Medical guidelines recommend that ultrasounds should only be performed when a valid medical indication is present, discouraging their use for purely non-medical purposes.
In a typical, healthy pregnancy, most women undergo a minimum of two ultrasound scans. The first is usually an early scan in the first trimester, often between six and twelve weeks, primarily to confirm the due date and viability. The second routine scan is the detailed anatomy survey performed in the middle of the second trimester, usually between 18 and 22 weeks.
Additional scans may be scheduled if concerns arise regarding fetal growth, the location of the placenta, or in cases of high-risk pregnancies. These subsequent examinations, often occurring in the third trimester, focus on monitoring fetal development and well-being.
Reasons for Additional Scans
- Fetal growth concerns
- Issues with the location of the placenta
- High-risk pregnancies, such as those involving multiple fetuses
- Maternal medical conditions like hypertension