As soon as that happens, the reproduction rate falls below the replacement rate.
The problem is that global society is actually not ready for a population that shrinks instead of grows. Many of our institutions are effectively pyramid schemes that rely on more people entering the bottom and paying for them than pull out money and exit from the top.
(Side note: I believe China is actually falling. There have been significant programs attempting to get women to have more children. Between education and "one child" (which is no longer in force) China has some demographic issues ahead.)