"二胎"的英文表达很简单，就是 Second child。例如：

1. Her second child is a girl.

如果你需要写一篇关于二胎的英语作文，可以参考以下例子：

Title: The Joy of Having a Second Child

In my family, there are only three people: my parents and I. Due to my parents' work, they could only have one child at that time, so I didn't have any siblings. I felt so lonely when my parents went out for their work. Now that the government has changed their policy, allowing people to have a second child, many families are embracing this new opportunity.

Having a second child brings many advantages. For one thing, the only child can have someone to company them, so they won't feel lonely during the process of growing up. For another thing, two kids can help each other and share things together, which fosters a stronger bond.

However, despite the expense, some people refuse to have a second child because they think they can't afford it. In conclusion, while having a second child can bring joy and companionship, it also comes with its own set of challenges and considerations.

希望这个例子能帮助你写出一篇关于二胎的英语作文。