永江 学

Manabu NAGAE

聖マリアンナ医科大学病院臨床検査部

Department of Clinical Laboratory, St. Marianna University Hospital

キーワード : Ultrasonography, Yolk sac, Embryology, Early pregnancy

　The anatomical location of the yolk sac is in the chorionic cavity, outside the amniotic cavity containing amniotic fluid and fetus. By ultrasonography the yolk sac is usually demonstrated in the gestational sac as a small circular structure distinguishable from the fetus.
　Ultrasound examination was performed on a total 763 pregnant women between 5 and 13 weeks of gestation, and the yolk sac was visualized in 321 cases (42.1%) at 6 to 13 weeks. The detection rate of the yolk sac was over 50% at 8 to 10 weeks and the highest was 56.0% at 8 weeks.
　The diameter of the yolk sac was from 2.1 mm to 9.9 mm and showed increasing tendency according to gestational age, gestational sac diameter and crown-rump length of the fetus, however, the variance was so wide that there were only poor statistical correlations between them.
　The rate of spontaneous abortion was higher in the group in which the yolk sac was not detected than in the yolk sac positive group. Especially, it is worth notice that, before 8 weeks of gestation, the possibility of the subsequent abortion was significantly higher when the yolk sac was not detected than when it was detected, even if the fetus echo or fetal heart beats was observed. Detection of the yolk sac by ultrasonography is useful to presume the prognosis of pregnancy before 8 weeks of gestation.