It's because women vary in the number of areolar glands that they have, and there is a correlation between the ease with which babies start to suckle and the number of areolar glands she has.
女性具有 不同的乳晕腺数目。 哺乳宝宝的容易程度 和母亲具有的乳晕腺数目 是正相关的。