I keep joking that I need to have another baby (NOT)
My weight fluctuated between 155 and 170 most of my early 30's and I got on the stick and got down to the 155 end before getting pg with dd.
Delivered at 194, got stuck at 186, worked it off and 10 more and kind of got stuck at 146 - got pg with ds (see avatar- isnt he cute

)
Ran and worked out like mad and delivered at...194. This time I dropped to 176 pretty quickly and then the REAL work began and I am puttering along now at 133 ish.
So one more baby and maybe I'll get stuck in the 120's?
Seriously, exercise and diet, its the same as with any other time in life. It came off slowly after dd and quickly after ds and I think the difference is the muscle I built working out while pg. But never too late to start!