OK, being the curious type that I am, I just had to try this out. I had 6 fish at various stages of pregnancy and put some growth hormone in the tank. Some babies appeared instantly, but not as many as would have been expected.

My guess is that since the growth hormone "jumps" the fish a certain number of days in age, whether or not they produce babies depends on how far they were from giving birth. If they turn the age when they would have given birth, you get instant babies. If they don't quite reach the age when they would have given birth, the babies will come sooner, but you still have to wait. However, if they pass the age when they would have given birth, they are deemed to no longer be pregnant. However, since the birth didn't take place, there are no babies in the tank. Hence, you lose the money for the babies and the money spent on growth hormone.