Should it be "by" instead of "in"?

I prefer "by" but "in" is also acceptable.

This medicine is not recommended for use by pregnant women unless absolutely necessary.

This is another way "in" could be used:

This medicine is not recommended for use in pregnancy unless absolutely necessary.

Is the following fine?

This medicine is not recommended for use during pregnancy unless absolutely necessary.

Yes, that sentence is the best of them all.