You need a baby to help solve a problem marriage.
Said no good friend, ever. Don't tell yourself this, don't listen to this kind of advice. A baby will only make things that much more stressful!
And it is really fair to put the burden of saving a marriage on a baby? No, it isn't.
If you have marriage problems please either seek counseling or a divorce, depending on how bad the problems are and if you think it can be fixed without adding an innocent baby into the mix.