https://www.newsweek.com/china-hiding-population-secret-1926834

The 2000 census came up short, so officials launched a campaign to add tens of millions of “missing” people, bringing the official population close to initial estimates, Yi said. But a closer look at demographics showed a glaring disparity, Yi said. Around 164.24 million babies were born between 1991 and 2000. After accounting for these births and subtracting deaths and net migration, there were about 40 million fewer Chinese than reported.

They killed so many girls the population fell more than expected.