How much do I have to gain for my BBL? I'm 5'5 137 lbs, 22.99 BMI, trying to get 40 " hips. (photos)

I generally do not believe weight gain pre-operatively is a good idea. This is true especially considering the fact that post-operatively some patients will struggle with losing the weight again. The optimum surgical candidate has reached a healthy and stable weight. 
This issue often comes up when discussing BBL and fat transfer. For these patients, adequate donor sites for harvest are necessary. The question of "adequate" will vary from one patient to the next. Your board certified plastic surgeon is uniquely suited to asses whether you have the reserves to meet your goals.
If you have reached a healthy stable weight (preferably with a BMI less than 30), the focus should be on maintaining. I typically do not recommend gaining/losing weight for the sake of surgery alone (weight loss for the sake of your health is a different story). It is important to remember that weight changes post procedure can potentially compromise the results of your buttock augmentation (as transferred fat can change with weight just as it could prior). If you plan on losing weight, I would do so before surgery and plan on maintaining afterwards. Weight gain should be discouraged unless you plan to maintain post operatively (or else the fat gained and transferred is typically the first to go post op).

Ultimately, decisions are based upon a patient's particular expectations. Some patients only want a subtle change and may be willing to accept a less significant result. But the rule still applies you have to have the fat to transfer the fat.

As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Thank you for your question. You appear to be a good candidate for a Brazilian butt lift. When planning a Brazilian butt lift, patients can look forward to a double benefit: reducing areas of unwanted fat while gaining a perkier, rounded posterior.  Each procedure must be planned according to the individual needs of patients, harvesting fat from target areas of the torso, thighs or knees and transferring it to create smooth, attractive contours optimized for each person. If you put on weight before surgery, there is more to transfer.  But I do not generally recommend this because we augment the buttock during surgery for optimal shape, and if you then lose the weight post operatively you will loose it disproportionately from your augmented buttock and the results could suffer.  We recommend patients to keep their normal weight and body form prior to surgery, and then we are working from a stable situation to create the changes we are working for.Buttock augmentation with fat grafting is one of our most popular procedures in Colombia, and from experience we can tell you that some patients do very well with 300 cc of fat per side while others can need 800 cc or more.  That said, you definitely have to choose your surgeon correctly and make sure that he/she has the training and experience to meet your goals with absolute safety.  Best is a Board Certified Plastic Surgeon with specialized expertise in this procedure.