child

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”。