Introduction
Ultrasound imaging has revolutionized obstetrics. The availability of this noninvasive diagnostic study, which produces no known harm on repeated patient exposure, has broadened the ascertainment of information about the condition of the fetus. That information can be used in concert with knowledge of the maternal status for obstetric decision making. This capability has, in the last three decades, significantly enhanced realization of the primary goal of obstetrics: a healthy outcome for both mother and baby.
Obstetric ultrasound imaging in the second and third trimester employs sound waves in the range 2–5 MHz (2–5 million cycles per second), transmitted into the mother’s body from a transducer held at her skin surface. Returning echoes are computer analyzed and processed, and the resulting two-dimensional image is displayed on a screen. Rapid data acquisition and processing permit the display of successive images with sufficient rapidity to create the impression that one is observing movement of the imaged structures in real time. The capability to observe movement has led to several important applications. Ultrasound imaging has been used in obstetric practice for fetal measurement, determination of fetal position, assessment of fetal well-being, guidance for procedures, and diagnosis of structural or functional abnormalities. It is well accepted, even requested and enjoyed, by patients.
Standardization of the technique and content of the obstetric ultrasound examination has been developed through clinical research and collective experience for the purpose of optimizing the accuracy of the information obtained from each clinical ultrasound examination. Standards are summarized in publications of professional organizations, for example the American Institute of Ultrasound in Medicine (AIUM): AIUM practice guideline for the performance of an antepartum obstetric ultrasound examination [1] (www.aium.org). The clinician is referred to these standards for a greater level of detail.
Content of the standard second- or third-trimester examination
A standard second- or third-trimester examination includes documentation of fetal life, assessment of the intrauterine environment and maternal structures, fetal measurements, and a fetal anatomic survey. A limited examination is applicable to specific circumstances, such as intrapartum management, guidance for certain obstetric procedures, and assessment of fetal well-being after a previous complete standard examination has been performed. A standard examination is expected to include documentation of cardiac activity, fetal movement, number of fetuses and the position of the fetus(es) within the uterus. Maternal structures should be described, noting uterine anomalies, abnormalities of the uterine tissues such as leiomyomata, and the size and appearance of the adnexal structures, if they can be visualized. The intrauterine environment is described, including amount of amniotic fluid, subjective or semi-quantitative using the Amniotic Fluid Index, location and appearance of the placenta, including its proximity to the internal cervical os. Measurement of cervical length may be included if clinically indicated.
The second- and third-trimester ultrasound examination includes, importantly, standard fetal measurements which are used to verify clinical dating or establish gestational age if clinical dating information is absent or inaccurate. Once gestational dating is established, the measurements are used to evaluate fetal growth. Generally, the ultrasound performed earliest in gestation would be expected to be the most accurate predictor of gestational age. A later examiner may occasionally judge that revision of the dating based on a subsequent examination is justified due to unavailability of documentation or the clinical circumstances of the initial examination.
Four fetal parameters are routinely measured in the second and third trimesters: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). Each parameter is measured by a specified technique and compared to a nomogram previously standardized on a well-dated pregnancy population [2–5]. Nomograms provided as charts for bedside comparison with measurement, or programmed into the software of the ultrasound equipment, display a mean ultrasound age associated with each standard measurement. If the mean age associated with each measurement is close to the known gestational age of the pregnancy, within 2 weeks, the gestational age and appropriate fetal growth are confirmed. If the measurements are discrepant with the gestational age or inconsistent with each other, a more detailed analysis is indicated. Measurement variation equivalent to plus or minus 2 weeks is a widely accepted and clinically useful estimate of accuracy applied to measurements in the middle trimester of pregnancy. Greater refinements of the estimates of variance for each parameter, if needed for research purposes, are available in the original literature reports [2–5].
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