From about 5.5 weeks, transvaginal ultrasound begins to visualize an extraembryonic structure in the fetal egg - the yolk sac (eng. yolk sac). At the same time, the beta-hCG level averages approximately 7200 IU/l.
Since the yolk sac is part of the embryonic structures, its detection makes it possible to distinguish a fertilized egg from a simple accumulation of fluid in the uterine cavity between the endometrial layers, and in most cases, makes it possible to exclude an ectopic pregnancy. The incidence of ectopic pregnancy is 1-2 in 2000–3000 pregnancies. Its risk increases with the use of assisted reproductive technologies (ART). It is necessary to suspect an ectopic pregnancy when the hCG level is more than 1500 IU/l, and the fertilized egg is not detected in the uterine cavity.