After thought - not original planed

The parents said that the baby was an after thought. 父母说生这个婴儿不是原来计划之内的。

Second thought - think it over , may change mind. 

Let me have a second thought. 容我再考虑一下！

Second opinion: He would like to have a second opinion from another doctor.  他要听听另一个医生的不同意见！