Prognostic significance of the human yolk sac assessed by ultrasonography.

Ultrasonographic examinations were conducted between 6 and 12 weeks' gestation in 77 first-trimester pregnancies with normal fetal outcome. Each examination consisted of measurements of the secondary yolk sac diameter and the fetal crown-rump length. The yolk sac was seen in all cases, and whereas its measurements demonstrated wide biologic variability, it correlated weakly (R2 = 0.39) with gestational age as confirmed by crown-rump length measurements. Growth of the yolk sac diameter, although slight, assumed a curvilinear relationship with gestational age. Such a growth profile is best described by a second-degree polynomial regression equation. The yolk sac performs important functions for embryonic development during organogenesis and the remnant of the secondary yolk sac seen on ultrasonography is often considered to be a potential predictor of fetal outcome. Our findings indicate that the size of this remnant in pregnancies with normal karyotypes and normal fetal outcomes is extremely variable. Additionally, the yolk sac size in patients with karyotypic abnormalities and spontaneous abortion were equally variable and almost all were within the normal range. In light of these findings, the secondary yolk sac size does not appear to be a sensitive predictor of embryonic integrity and pregnancy outcome.