CHAPTER 172 Obstetric Ultrasonography
Documentation
Adequate documentation for every ultrasonographic study is essential. This should include a permanent written report, complete with the ultrasonographic images incorporating measurement parameters and anatomic findings. Figure 172-1 is an example of an ultrasonography report form. Suggested documentation (adapted from the AIUM guidelines) for first-trimester, second- and third-trimester, and intrapartum scans is discussed in the following sections. Only standard obstetric ultrasonographic studies are discussed here, and they should include the elements described in the following sections.
First-Trimester Standard Scan Documentation
1 Document the location of the gestational sac. If visible, the embryo should be identified and the crown–rump length (CRL) measured and recorded. If an embryo is not visible, the mean gestational sac diameter should be recorded and can be used for estimating gestational age (Box 172-1).
2 Report the presence or absence of fetal life (e.g., cardiac or somatic activity). With transvaginal scanning, cardiac activity is usually observed when the embryo is 5 mm or greater in length; if not, a later scan may be needed to document cardiac activity.
5 Perform an evaluation of the uterus (including the cervix), adnexal structures, and the cul de sac for any abnormalities.
Second- and Third-Trimester Standard Scan Documentation
2 Report a quantitative and qualitative estimate of the amount of amniotic fluid (increased, decreased, normal; amniotic fluid index [AFI], single deepest vertical pocket, 2-diameter pocket).
3 Record the placental location and determine its relationship to the internal cervical os. The umbilical cord should be imaged and the number of vessels evaluated, when possible.
4 Assess gestational age using a combination of biparietal diameter (BPD; or head circumference) and femur length. Abdominal circumference can also be used to assess gestational age (see Box 172-1).
5 Assess fetal growth with the abdominal circumference measurements. If previous studies have been performed, give an estimate of the appropriateness of the interval growth. Fetal weight should be estimated in late second- and all third-trimester scans. It can be estimated from BPD, head circumference, abdominal circumference, and femoral diaphysis length. Significant discrepancies between gestational age and fetal weight may suggest the possibility of fetal growth abnormality, intrauterine growth restriction, or macrosomia.
7 The study should include, but not necessarily be limited to, the following fetal anatomy: head and neck, lateral cerebral ventricles, midline falx, cavum septum pellucidum, choroid plexus, cisterna magna, cerebellum, four-chamber view of heart, spine (cervical, thoracic, lumbar, and sacral), stomach (presence, size, and site), diaphragm, kidneys, urinary bladder, umbilical cord insertion site, number of umbilical vessels, extremities (presence and number), intactness of the anterior abdominal wall, and sex (medically indicated only in low-risk pregnancy with multiple gestations to possibly determine number of chorions).
Indications
• Suspected fetal death (see Chapter 225, Emergency Department, Hospitalist, and Office Ultrasonography [Clinical Ultrasonography])
• Intrauterine contraceptive device localization (see Chapter 225, Emergency Department, Hospitalist, and Office Ultrasonography [Clinical Ultrasonography])
• Premature rupture of membranes or preterm labor (e.g., estimation of fetal weight and/or presentation and/or cervical dilation)
Equipment
• Real-time ultrasonography machine with either a 3-MHz or higher transducer for transabdominal or transperineal scans (2 to 2.25 MHz may be needed for obese patients) or a 5-MHz or higher transducer for transvaginal scans (Among state-of-the-art machines, differences between manufacturers are primarily subjective [see the Suppliers section].)
Preprocedure Patient Preparation
Issues to be discussed with patients who undergo obstetric ultrasonography include the following:
A handout for the patient to review before scanning can be quite helpful (see the sample patient education form available online at www.expertconsult.com). After scanning, giving the patient a picture of the fetal hand profile, the facial profile, or even the genitalia should enhance bonding with minimal legal hazard.
Technique
1 For transabdominal scanning, the mother is usually most comfortable in the recumbent or semirecumbent position, tilted slightly to her left or with slight left lateral hip displacement. An adequate amount of gel should be applied to the abdomen. This position can also be used for transperineal scanning if the legs are flexed and wide apart. For transvaginal (and often for transperineal scanning), the mother is placed in the dorsal lithotomy position. An adequate amount of gel should be applied to the tip of the transducer before it is inserted into the sheath or glove for transvaginal or transperineal scanning.
2 By convention, transabdominal scanning is performed with the clinician on the right side of the bed, and the transducer position and image orientation are described relative to the mother (not the body of the fetus). When the marker dot of the transducer is located toward the mother’s head, a longitudinal (sagittal) scan is performed; the mother’s head will be located beyond the left side of the image (cranial direction) and her feet beyond the right side (caudal direction). When the marker dot is placed on the mother’s right side, a transverse image is produced; the mother’s right side will be located on the left side of the image and vice versa. (See Chapter 225, Emergency Department, Hospitalist, and Office Ultrasonography [Clinical Ultrasonography], for a more complete discussion of orientation.) By convention, transvaginal and transperineal scanning follow the same orientation for images.
3 For transvaginal scanning, additional gel is applied to the outside of the sheath and the transvaginal transducer is introduced into the vagina. Scanning starts as soon as the transducer is inserted; avoid inserting the transducer too far, which can cause the clinician to miss the cervix and lower uterine segment. The transducer handle is moved to the opposite side of the vaginal opening to evaluate the adnexa (e.g., the transducer handle is moved to the mother’s right side to evaluate her left adnexum).
4 For transperineal scanning, use the same transducer as for abdominal scanning. Additional gel is applied to the outside of the sheath or glove covering the transducer and the transducer is then placed against the introitus, labia, and perineum. During transperineal scanning, the vagina appears as a bright line usually meeting the cervix at a 90-degree angle. The distance from the perineum to the cervix usually places the cervix at an ideal distance for (and within) the focal zone of the transducer. Bowel gas in the rectum can sometimes obscure the external os. The image is sometimes improved if the patient is placed in the left-side-down decubitus position.
5 Follow a routine when performing a standard scan of an apparently normal pregnancy. However, even with a normal pregnancy you should have a low threshold for varying from the routine. For example, if you see an excellent sonographic view of something that will later need to be documented (e.g., placenta), freeze it and record an image. (You may not get another chance.) If an abnormality is noted, document it but do not forget to complete the routine scan.
6 First, briefly sweep the entire uterus to check for fetal viability and gross pathology as well as to determine the direction in which the fetus is lying. For first-trimester scans, it is important to methodically sweep the entire uterus to exclude multiple gestations. After gross pathology is excluded, it may enhance bonding to allow the patient and family to watch the images during scanning.
7 Next, with second- and third-trimester scanning, evaluate the lower uterine segment before the bladder fills and distorts the cervical length or its relationship to the placenta.
8 To maintain orientation, evaluate the long axis and transverse views of the spine if the fetus is in a convenient position. After getting oriented to the fetal spine, you will be three-dimensionally oriented to how the fetus is lying (e.g., on all fours). Transverse views of various organs will then be easier to obtain and will make more sense.
9 Evaluate the fetus in transverse views from head to pelvis. In particular, transverse views of the brain, spine, chest, heart, diaphragm, abdomen, stomach, kidneys, and bladder should be obtained. The cord insertion site should be imaged. Record appropriate images for documentation.
10 Longitudinal views of the spine, diaphragm, stomach, kidneys, and bladder should also be visualized. Record appropriate images for documentation.
11 Next, evaluate the extremities. The clinician should visualize all four extremities. Record an image of a femur for measurements.
12 A final sweep should be made through the entire fetus and at least an informal BPP performed for late-trimester pregnancies.
13 Finally, the placental site and amniotic fluid volume should be evaluated. For transabdominal scanning, if the placenta has not already been visualized during the previous scanning, it is usually located posteriorly. If there has been difficulty imaging the fetus, the amount of amniotic fluid is probably reduced.
14 While scanning, the necessary measurements listed in the next section should be obtained. Techniques and formulas for obtaining specific measurements are also discussed in the following sections. Use the appropriate sections when attempting to answer particular clinical questions or for certain situations as needed.
Measurements
The BPD, abdominal circumference, and femur length are measured as the basis of most obstetric ultrasonographic evaluations. Early in pregnancy, CRL and gestational sac measurements are also important. Certain early developmental landmarks, if noted, may also provide worthwhile information for estimating gestational age (Table 172-1).
1 CRL
• The formula is as follows:
TABLE 172-1 Developmental Landmarks According to Abdominal Ultrasonography*
| Landmark | Fetal Age (from LMP) |
|---|---|
| Visualization of gestational sac | 5–6 wk |
| Embryonic pole | 6–7 wk |
| Fetal heart motion | 7–8 wk |
| Fetal movement | 8–9 wk |
| Biparietal diameter measurable | 12–13 wk |
LMP, last menstrual period.
* Many of these may be visualized up to a week earlier with transvaginal scanning.
• The CRL is the longest length of the fetus in the sagittal plane and excludes the fetal limb buds and the yolk sac. One should average crown–rump measurements from three satisfactory images.
• Transabdominally, CRL is most accurate between 9 and 11 weeks. Even more accurate is CRL by transvaginal scanning between 7 and 9 weeks. The greatest problem with measuring CRL transabdominally earlier than 9 weeks is that the fetus is very small and the borders may be unclear. As a result, it may be very difficult to obtain the maximum longitudinal diameter (CRL) of the fetus. After 11 or 12 weeks, the fetus flexes and extends so much that it may be difficult to obtain the true maximal diameter (Fig. 172-2).
2 Gestational sac (GS) diameter
• The gestational sac measurement is not the best value to use for estimating gestational age, and it should be used only if other dating parameters are not available.
• The gestational sac consists of a hypoechogenic area, which corresponds to the chorionic vesicle, and an echogenic rim or ring, which corresponds to the trophoblast. The gestational sac of a normal pregnancy also may be characterized by a double echogenic ring. The inner ring is the decidua capsularis plus the chorion laeve. The outer ring is the decidua vera. At the implantation site, the hyperechoic rim is thicker, and it comprises the decidua basalis and chorion frondosum (Fig. 172-3).
• The in utero presence of a normal gestational sac, complete with contents (Fig. 172-4), usually confirms an intrauterine pregnancy and indirectly excludes ectopic gestation. In some cases, however, it may be difficult to differentiate between the gestational sac seen with an early intrauterine pregnancy and the pseudogestational sac sometimes seen with an ectopic pregnancy (see the First-Trimester Standard Scan Documentation section, and Chapter 225, Emergency Department, Hospitalist, and Office Ultrasonography [Clinical Ultrasonography]).
3 BPD
• The BPD is measured ideally when the fetus is lying in the occiput transverse position. In this position, BPD is the distance measured between the outer table of the proximal fetal skull and the inner table of the contralateral side of the skull.
NOTE: The BPD is one of the only outer-to-inner diameter measurements used in all of sonography. Inner diameter is used because the posterior calvarium causes a lot of artifact and tends to distort outer-to-outer diameter measurements (Fig. 172-5).
• The most commonly accepted reference plane for BPD is a cross-section parallel to the canthomeatal line and slightly above it. This cross-sectional plane cuts through the falx cerebri, the thalamus, the cavum septi pellucidi, and the medial cerebral artery. The head shape should be oval at this plane.
