Halle Berry did NOT make any deal with soon-to-be baby daddy Olivier Martinez over who gets custody and who pays for diapers ... so says her lawyer.

Celeb lawyer Steve Kolodny tells TMZ a report in Us Weekly is bogus ... a report claiming Halle and fiance Olivier have hashed out a prenatal agreement in which he agreed to pay her child support if they break up. Kolodny says not only is there no deal, there hasn't been a discussion or even a whisper about one.

TMZ broke the story ... Halle is a little more than 3 months pregnant with her second child -- predestined to be a very good looking boy.

What's more ... Kolodny says any agreement not ratified by a judge --  where one parent gives up the right to an unborn child would be unenforceable.