Hi S. - welcome to Mamapedia.
One quick suggestion before I answer your question: put a more descriptive title in your question like "toddler hits when she doesn't get her way" - you'll get better responses than when people don't know the topic.
Second - it's so much harder to deal with things when you are pregnant. You have one nerve left, and she's getting on it!
Toddlers don't have the verbal skills to express frustration - so they hit or bite or yell or throw things. That's completely normal. Frustrating, but normal. I'm not sure what techniques you have tried, but my suggestion is that you NOT try to reason with her. She's not even 2. She only understands her own "language" and her own reality, which is that if she does A, she gets B. If A is good, so is B. If A is bad, so is B.
So you do one of 2 things, depending on where you are. If you are at home and she hits, you deprive her of what she wants - her toys and her mom. You must do what is immediate - no delays like "no TV later" or "no dessert tonight" - kids under 6 just don't get that at all. So you pop her into her crib and you shut the door. She has nothing - no toys, no fun, but she is safe. If you are out, then you put her in the car seat as quickly as possible, no talking, no discussion except "We don't hit" or "we don't bite". That's all. If she wails and whines, fine. Put yourself where you cannot hear it - another room, or outside the car if you have to. I used to stand where my child could see me (so he didn't feel abandoned, just ignored) and I would read a book or do a Sudoku puzzle (kept a book in my purse for just this purpose). He learned pretty quickly that hitting = no fun.
So I don't know what you mean when you say you "discipline her" - whatever you are doing, it's not working if she's hitting all day long. You must make the consequences (not punishments) suitable. She's not yet 2, so figure 2 minutes tops. Then take her out of the crib/car seat and try again. Second offense? Back in the restrictive setting. If you have to do it 20 times, do it. You won't have to for more than a few days.
And remember that she's about to have her entire world turned upside down when the baby comes. So get this done now, so it's out of the way and not associated with the baby. You also want her under control and starting (just starting at this age) to use words (you'll have to help supply them for now - be brief and succinct) so she doesn't decide that hitting a baby is a good idea.
Mostly, you have to model self-control and not fly off the handle (as annoying as she can be) because you don't want her to do that too.
Good luck!