Hysterosalpingography questions

Ji

Hello, I was just wondering if any of you ladies have done a hysterosalpingography (HSG) test. My husband and I have been trying for baby #2 for about 18 months with no luck, no miscarriages or anything. My doctor suggested I do one of these to check to make sure my tubes are open. I guess I am just confused as to why they can’t see that in an ultrasound (which I have had a couple in the last few months). Anyway, any thoughts would be helpful!