The so-called “mid-embryonic period of development” occurs from 4 to 6 weeks from the beginning of the last menstruation. The anatomy of the embryo in this early period of development is usually still inaccessible for research due to the insufficient resolution of most diagnostic instruments currently used.
Differences in the timing of ovulation (up to 12 days) and implantation (up to 3 days) may influence the findings obtained during transvaginal examination at these early stages of pregnancy. With transvaginal echography, one of the first signs of intrauterine pregnancy is an anechoic round inclusion with a diameter of only a few millimeters, located in the uterine cavity against the background of a thickened hyperechoic endometrium. A gestational sac can be detected no earlier than 4 weeks and 3 days, but is most often detected by transvaginal examination after 5 weeks.
At 5 weeks of pregnancy, inside the fetal egg, surrounded on the periphery by a layer of hyperechoic chorion, two cavities are determined, located one inside the other, which represent the developing primary yolk sac and the cavity of the extraembryonic coelom (exocoelom). This picture is revealed only within 2-3 days. An embryo that is not visualized by echography at this stage is designated as three-layered, since upon microscopic examination three distinct cell layers (endoderm, mesenchyme and ectoderm) can be distinguished.
Since transvaginal echography is a relatively new clinical method, the need to accumulate additional clinical data remains relevant to establish reliable criteria for the relationship between the size of the ovum and visualization of the yolk sac and embryo.