A close look at early embryonic development with the high-frequency transvaginal transducer.

Transabdominal sonography has been, for the past two decades, used as an effective diagnostic and research tool in obstetrics. It is predominantly used in the second and third trimesters of gestation. Its use in the first trimester is relatively limited and mostly diagnostic in nature. The introduction of the higher frequency transvaginal transducer probe, with its higher resolution of the images, opens new possibilities to study early gestation. We studied embryonic development in 38 well-dated and normal pregnancies. A well-defined intrauterine gestational sac could be seen at 4 weeks and 1 to 4 days of menstrual age. The beta-subunit of human chorionic gonadotropin level at this time was 450 to 750 mlU/ml. Structures such as the yolk sac, membranes, ventricular system in the brain, musculoskeletal system, and cord were described and illustrated. Textbooks and atlases were used for comparative purposes. High-resolution transvaginal sonography will facilitate first-trimester perinatology.