A 35-year-old woman came to the family doctor with complaints of profuse menstruations that last up to 10 days. Gynecological examination shows that the uterine cervix is without changes, the uterus is in anterflexio, has normal size, is mobile and painless. The uterine appendages on the both sides are without peculiarities. The family doctor made the provisional diagnosis of abnormal uterine bleeding. What instrumental method of examination needs to be performed first to diagnose this pathology?