What is a ultrasound seen in 4 week 3 days of pregnancy?

Chorionic cavity: At 4 weeks 3 days gestation, via transvaginal us, a small fluid filled cavity, the chorionic cavity, can be seen. It's average diameter is 2-3 mm (tiny! look at a ruler). Surrounding it is an echogenic layer about 2 mm thick representing decidual reaction of the uterine lining and the trophoblasts of the developing embryo. The yolk sac is usually identified at 5.5 weeks and embryo at 6 weeks.