In early pregnancy, an ultrasound is performed to identify a viable embryo in the uterine cavity, confirm the gestational age, exclude pathology of the embryo, or identify normal variants, such as multiple pregnancies.
The initial sign of pregnancy is thickening of the endometrium, but ultrasound does not tell what exactly causes this thickening.
Using a high-resolution transvaginal transducer, a gestational sac with a diameter of 1 mm is visualized in the uterine cavity 4 weeks and 2 days after the last menstrual period during a regular menstrual cycle.
If menstruation is delayed for 5-7 days or more (gestational age is 5 weeks), a fertilized egg with a diameter of 6 mm should be clearly visible in the uterine cavity. It has a clear round shape with a fuzzy light rim along the periphery (hyperechoic rim - chorion). In this case, the level of beta-hCG in the blood is 1000-1500 IU/l. When the hCG level is more than 1500 IU/l, the fertilized egg in the uterine cavity should be clearly visualized.
With a lower level of hCG, the fertilized egg in the uterine cavity may not be detected by transvaginal echography. With a transabdominal examination, detection of the fertilized egg in the uterine cavity is possible at a beta-hCG level of 3000-5000 IU/l.