Fergie hasn't admitted to getting breast implants, so you'll have to draw your own conclusions from the before and after pictures. It looks like her breast size used to be smaller, but that's most likely a deception from her black shirt. Black clothes are good for hiding fat or in this case, a large chest.
This before and after photo shows Fergie from three different stages in her life. Her breasts look the biggest in the middle photo.
Again, it looks like she has too much cleavage for it to be natural. Also, her breast size increased by a solid cup size in the after photo.While the side angle is a little misleading, there's still a significant difference.
Do you think Fergie got a boob job?