We'd end up with government controlling the number of children, possibly even a lottery system. The overcrowding worries of the 70s and 90s were not because of a 1200% increase, rather a 10-20% increase.
And it would, if you knew you'd live that long most wouldn't tie themselves down with kids while they're still young, they'd wait much much longer and go out and live life.
As it stands now, if you don't have kids by the time you're 30 you risk not being able to have them at all and having to suffer though months of fertility treatments and such just to try.
People could want kids for a variety of reasons (to pass on their genes, the utility of having children, etc) and you'd have to maintain that you could only have two children max at one point in your life to effectively replace yourselves and not increase the population.
Not to mention if you'd want to have kids 2 or 3 times over, now that they're growth is only a small percentage of your total life.