Can I please get advice

I interviewed for a job they told me the interview process would take 4 weeks (background checks, reference checks and drug test) At the time I got the job offer I wasn’t pregnant after finding out I was pregnant they called for my start date for the job which is the same week as my ultrasound I’m so scared to tell my new job i have to leave early. I don’t want them to think I’m not serious or that they choose the wrong person. I talked to my Dr tried to get a later time but I’ll still have to leave work 30mins early