What happens to your boob job when you get pregnant?

Changes a bit. Because of hormone changes, you may get stretch marks, the breasts will sag as most large breasts do but the volume is likely to stay adequate. Breast glands atrophy after pregnancies and feel flat. Yours will keep better fullness.
See video. I have prepared a video that may answer many of your questions. This may depend on your breast feeding. The link is: http://youtu.Be/fybsihrevec.
Something. With out without breast implants, your breasts will change with pregnancy. There are many variations, but in average the breasts enlarge the skin stretches with lactation. Once laction stops, the breast volume decreases (of to small than before pregnancy) and the skin may not shrink to fit.
Breast sags. Like normal breast, the pregnancy causes enlargement of breast and then after pregnancy is over it leaves tell tale signs of stretch mark, sagginess etc.