Learn English – single word that means: doing something bad in the name of something good

These terms are holdovers from a previous era.

If a couple is trying to get pregnant, and the wife gets morning sickness, which potentially heralds wonderful news for the soon-to-be-parents, the wife's morning sickness is what we French refer to as "Un mal pour un bien," lit. out of bad comes good.

So, what sounds to come the closest here is "There is no rose without a thorn" or "It's an ill wind that blows no good."

There is no rose without a thorn: Prov. to enjoy any beautiful or pleasant thing, you must endure something difficult or painful.

It's an ill wind (that blows no one any good): A misfortune usually benefits someone.