There have been two theories:

- Since China controlled the pandemic via strict lockdown, where one cannot leave their apartments / housing compound for ANY reason, couples would be bored and get busy in bed to pass the time, resulting in a surge in birth rate.

- Also because of the strict lockdown, couples would be wary of having babies in such uncertainty. For example, if a woman gets pregnant and their compound goes into lockdown, she would have trouble getting prenatal care. Even if they can go to the hospital, they are afraid they will get COVID there.

So far it seems the second theory is closer to truth.