China's one child policy wasn't enforced well in the vast rural areas where the central government has less control over. I am speaking from own experience here. Born and raised in a city and being the only child of my parents, I was very surprised to see in my university that all my classmates from rural areas have siblings, and in most cases more than one. One of my roomates was the youngest son with 4 elder sisters, 3 of which never received higher than primary school educations and married early to bring back home some meagre income to support his education. In 1998 the flood, worst in the past 100 years, wreaked havoc in my province. Pai Zhou Wan 簰洲湾, a small town was completed destroyed. When the rescue team from the international red cross arrived there with more than enough supplies for the displaced 58,000 people registered as residents with the local government, they were shocked to find out that the actual population was 72,000. I believe since this flood, the international society started to wonder what the China's real population was.
By the way, China wasn't the only country which implemented mass birth control policy.
The article below is quite interesting.
"Coercion has been used in the West, too. The 1974 USA case of Relf v Weinberger showed that between 100,000 and 150,000 low income women were sterilised annually under federally-funded programs, and that some of these women had only consented to be sterilised after being threatened with losing welfare benefits"