Can fat cells expand post BBL?

Hello and Great Question.  I appreciate the time you are spending researching this procedure.  My name is Dr. Matthew J. Nykiel.  I am a Board Certified Plastic Surgeon, a Fellowship Trained Cosmetic Surgeon, and a Butt Augmentation Specialist.
The natural course of a BBL is get smaller over 4 weeks after surgery.  In my experience, the size of your butt will remain after 4 weeks from surgery.  I have noticed that around 6-12 months after surgery the buttocks will enlarge and "smooth" out after the fat cells establish.  However, in my experience, it is a dramatic increase in size.I hope that helps!  Best of Luck!
Matthew J. Nykiel, MD#TopREALSELFDoctor#CoreREALSELFDoctorButt Augmentation SpecialistLiposuction SpecialistBoard Certified Plastic Surgeon

Recovery from BBL is patient dependent. Much of this depends upon the amount of liposuction performed and the amount of fat transferred. I have observed that between 4 and 6 weeks the buttocks tend to stabilize in volume.  Usually by 3 months, the swelling is minimal and totally resolved by 6 months.  If you gain weight, you will gain where you have more fat cells. After a BBL, your butt  is one of the areas that will increase faster if you put on some weight.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

Dear SMJ2006,Fat viability depends a lot of surgeon technique. The fat has to be placed back into the buttocks as soon as possible so that it has a greater chance of survival. It has to be placed carefully so that not too much fat is placed in only one area. Most patients will lose 30-40% of transferred fat during first few weeks after the surgery. Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

Hi, SMJ2006! 
You are extremely early in your recovery at ten days. Early on your bum will look the largest because of the extra fluids present in the transferred fat and the trauma of surgery. Generally, the volume decreases as the body absorbs the excess fluid and absorbs whatever fat that has not established a blood supply. This process becomes stable, more or less between 3-6 months. That being said, if you gain weight some of that will go to your buttocks but it is variable how much. Other factors include how the fat was processed and how much was transferred. Try not to despair, it looks like you have a nice result. If the size difference bothers you after that 3-6 month period, bring it up with your plastic surgeon. I'm sure a remedy can be figured out. 
Hopes this helps,
Dr. M

Multilayered micro-fatgrafting to the buttocks is the key to successful Brazilian butt lift surgery. Transferring purified fat cells mainly into the fascia and muscle layers with a very fine cannula (approx. 0,9mm diameter) is an essential step to assure best possible take of grafts. Invariably, this will lead to some degree of swelling.
What you are seeing as fat lose or shrunk is really swelling going down.
It'll take more than 6 months to completely recover.
Good Luck!