Is it true that once the fat is taken from an area it can't grow back in that same area when you gain weight? Considering BBL.

Thank you for your question. To provide a valid medical opinion I will need to see pictures of you body from different angles. I would suggest to be treated by board certified plastic surgeon with experience in this procedure, to get the best desired results.
Respectfully,Dr. Emmanuel Mallol Cotes.-

Liposuction in one area depletes fat cells in that area.  As a result, if a patient gains weight, the fat will be stored in other areas of the body preferentially.  Liposuction in an area reduces the number of cells in that area and the propensity for gain in the future.  Fat will be less likely to return to the places that have been liposuctioned. Fat cells can always increase in size with weight gain. Hence, the reason for weight maintenance.The BBL procedure forever alters the distribution of fat in a favorable way. Pregnancy after these procedures is not dangerous, but a second TT may be required to repair muscle laxity or loose skin after all your pregnancies. Pregnancy and delivery should not pose an additional challenge, but this depends on a number of things including placement of incisions for the TT, and whether a C-section is performed. You should not worry as your obstetrician will be experienced in handling any scar tissue.
Kenneth Hughes, MD, Board Certified Plastic Surgeon
Los Angeles, CA

After puberty, an individual will have the same population of fat cells for the remainder of their lives (barring an intervention). Any changes in weight will manifest not as more or less fat cells but rather swelling or shrinking of those very same fat cells.
Predicting, with any measure of certainty, the effect of weight loss on the visceral and subcutaneous fat populations is not possible. There are differences with regards to sex with males demonstrating a predisposition to the accumulation of visceral fat. Beyond this, the best that can be offered is a guess.
It remains unclear how the body directs particular populations to change post liposuction. Patients will often lament that attempts at weight loss are frustrated by losses in areas not targeted and weight gain by contrast goes directly to perceived problem areas. It is impossible to predict with certainty how a patient will respond to liposuction, however, it is a reasonable assumption that treated areas demonstrate a reduced potential for weight gain. 
The best advice one can give is to reach a stable healthy weight pre-operatively and try to maintain post-operatively. This is the best way to achieve a good result with the best chances for longevity.With regards to your question, the results of pre-pregnancy surgery can be compromised by the changes associated with having children (especially contouring/tightening procedures such as a "tummy tuck"). While this is not a reason to avoid surgery, you should be comfortable with the potential need for revisions. 
As always, discuss your concerns with a board certified plastic surgeon (ABPS).