Some of those factors might be the use of birth control pills, earlier menstruation, rising rates of obesity, more women smoking, and later ages at first pregnancies.

• 其中部分原因可能与使用避孕药，较早出现月经初潮，肥胖比例增加，更多妇女吸烟，以及高龄的初次怀孕等有关。