Overview of Obstetric Imaging

First Trimester

The current guidelines of the American College of Radiology (ACR) and American Institute of Ultrasound in Medicine (AIUM) for the performance of first-trimester obstetric ultrasound examination include documentation of the location of the pregnancy (intrauterine vs. extrauterine), documentation of the appearance of the maternal uterus and ovaries ( Fig. 28.1 ), and assessment of gestational age, either by measurement of mean sac diameter (before visualization of embryonic pole; Fig. 28.2 ) or by embryonic/fetal pole crown-rump length ( Fig. 28.3 ). Another important structure to assess is the yolk sac. An image of the heart rate is taken using M-mode ultrasound. It is important to use M-mode rather than spectral Doppler ultrasound on the embryo to limit power deposition. Late in the first trimester, dating can be performed with measurement of the biparietal diameter and head circumference rather than crown-rump length.

Videos 28.1 and 28.2 show normal first-trimester findings of an early embryo with cardiac activity (Video 28.1 ) and normal findings of rhombencephalon (Video 28.2 ).

In the first trimester it is important to not only establish the location of the pregnancy (intrauterine versus extrauteruine) but when intrauterine, to carefully determine if it is a potentially viable pregnancy or if it is a nonviable pregnancy. Due to the variety of medical professionals performing and interpreting ultrasound in a variety of clinical settings, thresholds for the diagnosis of a failed pregnancy have been increased in order to not misdiagnose a potentially viable pregnancy. These issues are discussed in detail in Chapter 30 .

In cases of multiple gestation, first-trimester scans should document the fetal number as well as the amnionicity and chorionicity ( Fig. 28.4 ). Chapter 32 discusses the assessment of multifetal pregnancies.

In addition, the American College of Obstetricians and Gynecologists (ACOG) recommends that prenatal testing for aneuploidy be offered to all pregnant women. Clinicians must understand current screening options (and the tradeoffs between them), including traditional serum analysis (with or without nuchal translucency ultrasonography) or cell-free DNA so that they can discuss these options appropriately with the patient. Cell-free DNA testing uses DNA from the placenta to assess the risk of the fetus having a chromosomal abnormality; it does not assess risk for fetal anomalies such as neural tube defects or ventral wall defects. Management decisions, including termination of the pregnancy, should not be based on the results of the cell-free DNA screening alone. Patients should be counseled that a negative cell-free DNA test result does not ensure an unaffected pregnancy. According to a 2015 ACOG committee opinion, “Given the performance of conventional screening methods, the limitations of cell-free DNA screening performance, and the limited data on cost-effectiveness in the low-risk obstetric population, conventional screening methods remain the most appropriate choice for first-line screening for most women in the general obstetric population.”

Thus despite increased use of cell-free DNA testing, ultrasound screening is still being performed by measuring nuchal translucency between 11 and 14 weeks of gestation (see Fig. 28.3I ). This measurement, in conjunction with maternal age and serology, can be used to determine an individualized risk of fetal aneuploidy (see Chapter 31 ). Increased use of maternal serum screening, as well as first- and second-trimester ultrasound have reduced the number of interventional procedures to detect aneuploidy while increasing the number of prenatal diagnoses of aneuploidy. Given the increased scanning late in the first trimester, it is also increasingly common for a limited anatomic survey to be conducted in the late first trimester. Anomalies that should be detected this early include anencephaly ( Fig. 28.5 ) and omphalocele ( Fig. 28.6 ). Although substantial information can be obtained at this time, the first-trimester anatomic survey is unlikely to replace the second-trimester anatomic survey, since many structures are difficult to visualize completely early in the second trimester, particularly the heart, cardiac outflow tracts, posterior fossa, and distal spine.

Second and Third Trimesters

The current ACR/AIUM guidelines for the performance of the second- and third-trimester obstetric ultrasound examinations describe the standard sonographic examination. It is important to understand that the guidelines were written to maximize detection of many fetal abnormalities but are not expected to allow for detection of all structural abnormalities.

The terminology level I and level II examinations refer to “standard” or “routine” (level I) and “high risk,” “specialized,” or “detailed” (level II) obstetric ultrasound. The concept of these two levels of scanning is that the standard, basic, routine, or level I examination is performed routinely on pregnant patients ( Figs. 28.7 to 28.15 , Videos 28.3 ) to 28.9 . The methods to obtain all the required images are described in detail in subsequent chapters. This chapter provides a collage of figures as a guide for the anatomic survey and common additional views obtained during a fetal survey.

In general, the “standard fetal anatomic survey” refers to the second-trimester scan, typically performed between 16 and 22 weeks of gestation. When anatomic surveys are performed at 20 to 22 weeks' gestational age, there is less need for repeat scans to document normal anatomy compared to studies performed earlier in pregnancy. However, there are practical considerations when determining the optimal timing of studies. In well-dated pregnancies in women who are unlikely to want amniocentesis, a survey at 20 to 22 weeks' gestation is optimal. However, if a pregnancy is not well-dated, an earlier scan may be needed both to establish accurate dates for the pregnancy and to assess the anatomy. Some centers offer the scan at 16 weeks' gestation to coincide with performance of genetic amniocentesis and/or midtrimester quadruple serum screening.

The level I examination consists of investigation of the maternal uterus and ovaries, the cervix, and placenta ( Fig. 28.7 , Video 28.3 ), as well as a systematic review of fetal anatomy. Adnexal cysts are common in pregnant women. In early pregnancy a cyst is most likely the corpus luteum. If a cyst appears atypical or enlarges beyond the middle second trimester, it should be further assessed. Leiomyoma position and size should be documented. If the myometrium appears thin in the lower uterine segment (e.g., <3 mm in a woman with prior cesarean section), the myometrium should be measured with transvaginal sonography and should be followed later in pregnancy because the thin myometrium puts the woman at risk for uterine dehiscence and/or rupture. It is helpful to begin the examination with a sagittal midline view to assess the cervix. If the cervix appears abnormally short or if placenta previa is suspected, a vaginal scan can then be performed.