Using the results obtained from examining patients who were treated with in vitro fertilization (IVF), it was found that the fertilized egg is usually visualized between the 4th and 5th weeks after the last menstrual period. Experience shows that it can be detected after 4 weeks and 2 days, counting from the first day of the last menstruation. In most cases, with transvaginal echography, the yolk sac is visualized when the size of the fertilized egg reaches approximately 1 cm, and the yolk sac together with the embryo - when its size is about 1.5 cm.
Based on preliminary data, it was revealed that during the period of detection of the ovum during transvaginal echography, the level of beta-hCG ranges from 500 to 800 mIU/ml according to the criteria of the Second International Standard, and according to the new criteria it ranges from 1,500 to 2,400 mIU/ml, which significantly lower than those found when compared with the results described for transabdominal echocardiography (1,800-3,000 mIU/ml).
During this period, the fertilized egg itself increases in diameter by approximately 1-2 mm daily and is determined within the thickened endometrium. To record the dynamics of the size of the fertilized egg, a repeat study can be performed after 3-5 days. In the middle of the 5th week from the last menstruation (3.5 weeks of gestational age), the embryo, whose dimensions range from 2 to 5 mm, is located near the wall of the yolk sac, which is visualized as a round cystic structure with anechoic contents with a diameter of 2 to 4 mm. An increase in the yolk sac (over 6 mm), as well as its decrease, may indicate the death of the embryo. The embryo with the yolk sac are located eccentrically in the cavity of the fertilized egg, forming an image of the so-called “double bubble”.
By the end of the first half of the embryonic period, the chorion surrounding the fertilized egg is visualized as a ring of hyperechoic tissue. At 4 weeks from the last menstrual period, the size of the ovum is only 3 to 5 mm in diameter, increasing to approximately 1 cm by the 5th week of pregnancy.
In the early stages of development, the embryo may be difficult to visualize during transvaginal examination. Despite the fact that many of its structures have already been formed, they have not yet been identified using echography. During these periods, the neural tube closes in the middle part, but remains open in the region of the head and caudal ends. Gill arches are formed, as well as somites, which look like rounded elevations on the surface of the embryo. Until the end of this period, 42-44 pairs of somites are formed, from which skeletal bones and muscles will subsequently develop.