The discriminatory level for a yolk sac with transabdominal ultrasound was a mean gestation sac diameter (MSD) of 20 mm.9 Early TVS described a discriminatory level for the yolk sac of 8 mm.10 More recently Rowling et al.11 found that with 5 MHz transducers the yolk sac is definitively seen by a MSD of 13 mm but with newer 5-9 MHz transducers a yolk sac was definitively seen by the time the MSD was 5 mm. The presence of a larger sac with no evidence of yolk sac formation contained within it is a definitive sign of a non-viable pregnancy (Figure 4.4) In addition an abnormal or hydropic yolk sac is a non-specific sign or 'soft marker' of early pregnancy failure (Figures 4.5 and 4.6).
Figure 4.5 A highly magnified view of a gestational sac in a woman at 6+4 weeks by her LMP. This scan shows a small sac with a large hydropic (8.5 mm) yolk sac. This picture is compatible with intrauterine pregnancy failure.
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