Hi, Gissele.

   I will post some ways of speaking, sometimes more informal and other times less.

   Up to you to tell one from the other, okay? homework, he hee.   A little hint, using  gender it´s more formal, "sex" is a more day-to-day (say, of the street) language.

   The date (tomorrow) is on my own, because most of pregnant women mark the days in anticipation...

   -You would say "Tomorrow I am going to have a sonogram to know the baby sex/gender"

   -Tomorrow at 12:45 is my ultrasound to know the baby sex...
  

   -I am scheduled to have an ultrasound the day after tomorrow/tomorrow/next week/next Monday etc.
   

   -Day after tomorrow I am having an ultra sound because I want to know the baby sex.

   -I have to see my doctor and get another requisition to book another ultrasound to
know the baby sex, I hope he send me in for an ultrasound tomorrow. I barely can´t wait!

   -I am going for an ultrasound tomorrow, ´cause I want to know the sex of the baby.

   These are "rough diamonds", up to be polished by more advanced learners, let´s wait then.