Of biometric parameters, AC is most afected by fetal growth. Thus, for gestational age estimation, AC has the greatest variation, which can reach 2 to 3 weeks in the second trimester. To measure the AC, a circle is placed outside the fetal skin in a transverse image that contains the stomach and the confluence of the umbilical vein with the portal sinus (see Fig. 10-2C). The image should appear as round as possible and ideally contains no more than 1 rib on each side of the abdomen. he kidneys should not be visible in the image.
Variability of the sonographic gestationl age estimate increases with advancing gestation. Accordingly, pregnancies not imaged prior to 22 weeks to conirm or revise gestational age are considered suboptimaly ated (American College of Obstetricians and Gynecologists, 2017a). Although the estimate is improved by averaging multiple parameters, if one parameter difers
TABLE 10-2. Some Indications for First-Trimester Ultrasound Examination
Define the cause of vaginal bleeding
Assist chorionic villus sampling, embryo transfer, and
Assess for certain fetal anomalies, such as anencephaly, in a bnormaIities Measure nuchal translucency /hen part of a screening
Modified from the American Institute of Ultrasound in significantly from the others, consideration should be given to excluding it from the calculation. he outlier may result from poor visibility, but it could also indicate a fetal abnormality or growth problem. Reference tables such as the one in the Appendix (p. 1264) may be used to estimate fetal weight percentiles.
Indications for sonography before 14 weeks' gestation are listed in Table 10-2. Early pregnancy can be evaluated using transabdominal or transvaginal sonography, or both. he components listed in Table 10-3 should be assessed. First-trimester sonography can reliably diagnose anembryonic gestation, embryonic demise, ectopic pregnancy, and gestational trophoblastic disease. he irst trimester is also the ideal time to evaluate the uterus, adnexa, and cul-de-sac. Determination of
TABLE 10-3. Components of Standard Ultrasound Examination by Trimester
Gestational sac size, location, and number Embryo and/or yolk sac identification Crown-rump length Fetal number, including amnionicity and chorionicity of the first rimester Evaluation of the maternal uterus, adnexa, and cul-de-sac Evaluat"on of the fetal nuchal region, with consideration of fetal :chal translucenry assessment
Fetal number, including amnionicity and chorionicity of multifetal gestations
Placental location, appearance, and relationship to tie internal cervical os, with documentation of placental cord insertion site when technically possible
Fetal anatomical survey, including documentation of technical limitations
Evaluation of the maternal uterus, adnexa, and cervix when
Modified from the American Institute of Ultrasound in Medicine, 2013a.
chorionicity in a multifetal gestation is most accurate in the first trimester (Chap. 45, p. 868).
n intrauterine gestational sac is reliably visualized with transvaginal sonography by 5 weeks, and an embryo with cardiac activity by 6 weeks (Fig. 10-3). The embryo should be visible transvaginally once the mean sac diameter has reached 25 mm-otherwise the gestation is anembyonic. Cardiac motion is usually visible with transvaginal imaging when the embryo length reaches 5 mm. In embryos <7 mm without cardiac activity, subsequent examination may be needed to determine viability (American College of Obstetricians and Gynecologists, 2016). At Parkland hospital, first-trimester demise is diagnosed if the embryo has reached 10 mm and lacks cardiac motion. Other criteria for this diagnosis are found in Chapter 18 (Table 18-3, p. 350).