At 5 to 6 weeks' gestation, a 1-to 2-mm embryo adjacent to the yolk sac can be seen (Daya, 1993). As listed in Table 18-3, absence of an embryo in a sac with a mean sac diameter (MSD) ::25 mm signiies a dead fetus (Fig. 18-1). Fetal cardiac activity can typically be detected at 6 to 6.5 weeks with a crown-rump length (CL) of 1 to 5 mm and an MSD of 13 to 18 mm (Goldstein, 1992; Levi, 1990). A threshold CL ::7 mm with absent cardiac activity is also used to diagnose nonviability (Doubilet, 2013). Preisler and associates (2015) implemented the guidelines in Table 18-3 and conirmed' these CL and MSD thresholds. However, for cases in which a gestational sac had no embryo or yolk sac and measured < 12 mm, they
TABLE 18-3. Guidelines for Early Pregnancy Loss Diagnosisa
CRL ::7 mm and no heartbeat MSD ::25 mm and no embryo An initial US scan shows a gestational sac with yolk sac, and after ::11 days no embryo with a heartbeat is seen An initial US scan shows a gestational sac without a yolk sac, and after ::2 weeks no embryo with a heatbeat is seen
Transvaginal preferable to transabdominal US
M-mode imaging used to document and measure heartbeat Doppler US not used to evaluate a normal early embryo aFrom the Society of Radiologists in Ultrasounds; American College of Radiology.
CRL = crown-rump length; MSD = mean sac diameter; US = ultrasound.
From Doubilet,
Lane, 2013.
recommended that to diagnose nonviability after 2 weeks, in addition to the lack of an embryo with a heartbeat, the MSD should have failed to double.
During scanning, because of theoretical temperature elevation in tissues exposed to pulsed Doppler beam, this modality is applied only when needed for additional diagnostic purposes. M-mode should be used to document cardiac activity and measure the rate (Lane, 2013). Finding of an IUP and cardiac activity lowers subsequent miscarriage rates (Siddiqi, 1988).
In addition to the diagnostic parameters of Table 18-3, other softer sonographic markers may portend early pregnancy failure. Values for yolk sac diameters (measured inner-to-inner ring) for each gestational week in normal pregnancy have been established. Yolk sac diameters ::6 mm in pregnancies < 10 weeks' gestation are suspicious for pregnancy failure (Berdahl, 2010; Lindsay, 1992). The fetal heart rate in the irst trimester rises from 110 to 130 beats per minute (bpm) at 6 weeks' gestation to 160 to 170 bpm at 8 weeks (Achiron, 1991; Rauch, 2009). A slower heart rate is unfavorable, especially those <85 bpm (Laboda, 1989; Stefos, 1998). Even with cardiac activity, fetuses with a small MSD may presage embryonic loss. Speciically, a diference <5 mm between the MSD and CRL values raises concern (Bromley, 1991; Dickey, 1992). Last, subchorionic hematoma, that is, blood collected between the chorion and uterine wall, often accompanies threatened miscarriage. Studies are contradictory as to its association with ultimate pregnancy loss (Pedersen, 1990; Stabile, 1989; Tuuli, 2011). Bennett and associates (1996) noted that miscarriage risk correlated with larger hematoma size, older maternal age, and bleeding at a gestational age :;8 weeks.