You are a great candidate for a subtle BBL but a great improvement in you. Pregnancy will not affect you any differently than it would have without the surgery.
You could do a BBL or implants - what we decide would depend on your goals, so bring dream pictures with you so we can see pics of what you want. If I can't get enough fat out via lipo to meet your goal, then you'd need an implant. Easy discussion to have with your PS. Pregnancy won't really affect your results per se... If your weight fluctuates, the fat in your bum will fluctuate some too. But it won't get smaller considering how thin you are. An implant obviously won't fluctuate. Typical range for this would be 4-10k. I can't tell for sure without seeing you in person.
The truth is "life happens to all of us." We gain weight and lose weight. We may have kids. We are all genetically programmed to change as we age. Plastic surgery does not change that; it just provides an improved starting point for life's continued changes. If you had a Brazilian, the distribution of your fat would be altered. Some body sites would have fewer adipocytes, and your bottom would have more. Consequently the pattern of weight gain and weight loss would be different than if you didn't have a BBL. The truth is, no one can tell you how you will look after pregnancy. Some women's bodies are dramatically changed by a single gestation with a small baby and other people are like Heidi Klum. I hope you have genes like Heidi! I would encourage consultation with a Board Certified Plastic Surgeon to discuss the matter further, and also to assess whether you have enough donor fat for a BBL. Good luck!