with child/with a child

【问】 上面两个介词短语意思是否相同?

【答】 上面两个短语的意思是完全不同的。with child是个成语，意思是“怀孕”：

Three months after her marriage Mrs Long learned that she was with child. (Whitford)朗太太在她结婚三个月之后得知她已怀孕了。

with a child是个自由组合的短语，意思是“带着一个孩子”。