No matter what the disagreement is about, these four basic rules will help you cope. And remember — the conflict resolution and compromise you're practicing now will be even more important once baby comes. Treat these discussions as learning opportunities!
No pressure or guilt trips
Rather than attempt to wear down your partner with endless appeals (never works), try to accept his different opinion, then revisit the topic once you've both had time to consider compromises. Remember, marriage is already a huge change. Give your spouse a chance to breathe before pushing another on him.
Be open and honest
This means no "forgetting" the birth control. Tricking him into such a major, life-changing undertaking is a manipulation that will put a sour note on your entire marriage and parenting partnership. Your spouse should welcome your child, not resent you for having it.
If the disagreement seems insurmountable or is affecting other areas of your marriage, there's no shame in meeting with a marriage counselor. Sometimes, it takes a third party to work through the issues and find an acceptable solution.
Focus on what's important now, and let the other stuff go. So you can't agree on whether to have two or three kids... think maybe that can wait until after the first one comes? Chances are, it'll make you both rethink how you feel about further additions to the family. Start with your first baby, and take the rest as it comes.