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Ultrasound imaging methods are based on isonating target tissues with low-energy (<100 MW/cm2), high-frequency (3.5-7.5 MHZ) sound waves and recording the intensity and the delay time for reflected sound. There is an increased incidence of abnormalities resulting from in utero exposure to ultrasonography, and long-term follow-up studies have shown problems.
Types of Examinations
Current ACOG guidelines describe two types of obstetric ultrasound examinations: basic and comprehensive examinations. A basic examination is intended to be a thorough, well-documented examination performed for any of the usual indications. A comprehensive examination may be indicated. Brief or limited examinations may be appropriate during emergencies or when ultrasonography is used as an adjunct to a procedure. Ultrasonography is also used to provide guidance during amniocentesis, chorionic villus sampling (CVS), and percutaneous umbilical cord blood sampling (PUBS).
BASIC EXAMINATIONS
The components of a basic examination during the second and third trimesters are determination of gestational age; fetal number, viability, and lie; placental location and grade; and volume of amniotic fluid. The examination also includes a survey of fetal anatomy for gross malformations.
First Trimester. In the late first trimester, fetal anatomy can be seen in the fetus but occasionally is difficult to interpret. For example, it is not unusual to see the physiologic herniation of bowel into the umbilical cord as late as 10 weeks of gestation. This finding may be mistaken for the early diagnosis of an omphalocele.
During a basic examination in the first trimester, gestational age is estimated and the number of fetuses is recorded. This may be difficult in the very early first trimester. In a case of monochorionic twins, two sacs may not be visible until the second trimester. If the fetal pole cannot yet be distinguished, a set of twins may be missed in this circumstance. A phenomenon known as vanishing twins involves up to 15% of twin.
Second and Third Trimesters. The basic examination in the second and third trimester is similar to that in the first trimester. The number of fetuses, cardiac motion, and lie should be documented. Multiple parameters are measured, usually including the femur length, abdominal circumference, head circumference, and biparietal diameter. In the second trimester, combinations of these parameters may predict gestational age to within 10 days. A 2-week error is easily made in these measurements.
Evaluation of Structures. The umbilical cord should be examined briefly in a basic examination. The number of vessels contained in the cord can be seen after 24 weeks and frequently before this time. A single umbilical artery may be associated with fetal anomalies, particularly in the genitourinary system.
Amniotic Fluid Volume. Amniotic fluid volume can be estimated during an ultrasound examination as normal, abnormally increased, or abnormally decreased.
A reliable way to describe amniotic fluid volume after 20-22 weeks is the amniotic fluid index.
Fetal Anatomy Survey. For a basic examination to be complete in the second and third trimesters, a variety of fetal structures should be imaged. These structures include intracranial anatomy (cerebellum, lateral ventricles, and cisterna magna), the spine, all four chambers of the heart, the stomach, insertion of the umbilical cord into the anterior abdominal wall, and the kidneys.
COMPREHENSIVE EXAMINATIONS
A comprehensive examination is generally indicated when suspicion has been raised about the pregnancy or the fetus. A common indication for a comprehensive examination is an abnormal result on basic examination. Risk factors include a history of congenital anomalies, abnormal alpha-fetoprotein test results.