only child（男女都可以）复数：only children

the only child in one's family 都有 独生子女 的意思。不过多用only child.

singleton 也可以，不过就是独生子的意思。

例：过分溺爱独生子女的父母

Parents who dote on their only child.

独生子女有时被宠坏了。

Only children are sometimes spoilt.

而extra births就是“超生人口”，也就是我们平时说的“计划外生育”，也可以称为extra children /additional births。根据family planning policy（计划生育政策），一般情况下每个家庭只允许生育一个孩子，也就是我们所说的only/single child（独生子女）。

不过要介绍的是下面这个表示方法：China one.

　　The term “China one”refers to the child who was born after the 1980s, when China launched the one-child policy. Not only are they the family’s only child, but they are also the family’s center of attention. Their parents and grandparents would spare no efforts in taking care of them, and would give them anything and everything, as if they were the “emperor” of the family.

　　China one指的是20世纪80年代中国实行计划生育政策以后出生的独生子女。他们是家中唯一的孩子，也是家庭的中心。他们的父母和祖父母都会不遗余力地照顾他们，满足他们的一切需要，就好像他们是家里的“皇帝”一样。

　　As a result, most children of the “China one”generation lack independence as they have become so reliant on their families for support. Some parents even hired a maid for their children when they go to college. Other unfavorable characteristics include: bad communication skills, rudeness, ungratefulness and self-centeredness.

由此，大多数独生子女自理能力都很差，他们太过依赖家人，甚至上大学以后还有父母为他们雇保姆。其它不太让人喜欢的特点还有：沟通能力差、不懂礼貌和感恩、以自我为中心等。