MFM vs OB?

This might be a dumb question, but what’s the difference between an OB and an MFM? I always hear about girls who are high risk going to an MFM high risk dr.

I have Adenomyosis and I had Preeclampsia and HELLP syndrome with my first baby, born via emergency c-section. My platelets are still low from my HELLP syndrome (130s) and my liver is permanently damaged, but functional obviously. I’m also over 35 and used IVF to get pregnant.

I’m 5 weeks pregnant. When I go for my first appointment with my OB am I supposed to ask about seeing a high risk dr, or will my OB automatically refer me? Or is an OB the same as a high risk dr? Is there anything else I should know? Thanks!