The antonym or opposite word for before is after.
before
antonym of after
Prenatal would be a good antonym for antenatal.Antenatal means 'after birth', prenatal means 'before birth.'
The prefix "ir-" before "regular" would create the antonym "irregular."
There are a few antonyms for the word before such as after, later, behind and afterwards.
Forenoon This means morning. It is not in common usage, but is a direct antonym of afternoon, meaning "before noon".
previous, prior, initially, before, etc.
An antonym for the postmeridian- meaning a time pertaining to the afternoon, is premeridian- which means time relative to the morning. Although, in simplistic terms, saying an antonym for postmeridian is 'morning/time before noon' will suffice.
The antonym of ego is humility. Humility involves a modest view of one's own importance and a willingness to consider others before oneself.
The antonym of the word 'synonym' is 'antonym'.
== == Beginning, first, front, initial, introductory, oldest, primary, starting