After a BBL, there will be some swelling on the buttocks, that slowly goes away. In many cases, the patient thinks that the fat is disappearing, but most of the volume lost, is from the swelling that is going away, and usually in the first month is when the patient notices the biggest change in volume, and then in the following months the amount of change is much less. Not all the injected fat survives, and also the percent of survival is different among patients. In my practice about 30 to 40% of the total injected volume goes away 3 months after the surgery. To preserve the blood supply to the injected fat, the patient should avoid any pressure on the buttocks, for at least 2 weeks, that it is the time, the injected fat needs to get new supply and survive. Even after that, it is recommendable to avoid sitting on the buttocks, for long periods of time during the first 4 weeks, so sitting on a memory foam pillow it a good idea, and taking breaks from sitting, every 15 minutes is another way to avoid too much pressure on an specific are of the buttocks. Check with your surgeon, for specific instructions.