Maybe what's really bothering you is the fear of the unknown. Try a childbirth education class or read a book specifically geared toward dads. Talk to your partner's obstetrician, midwife or prenatal instructor. There's a good chance that the more you know about what lies ahead, the more comfortable and confident you'll feel.
Discuss your feelings with your partner.
She needs to know what you're feeling and why, and it can help to find out exactly what she expects from you during labor and delivery. But be particularly sensitive to the way you do this. You don't want your partner to interpret your apprehensiveness as a sign that you don't care about her or the baby.
Clarify your role.
Maybe you really don't mind being there for the birth, but you're concerned about what may be required of you while you're there. Not every father has to cut the umbilical cord, or videotape the entire blessed event. Although this is your partner's big day, you do have the right to draw the line at certain things that you're truly reluctant to do. Talk with you partner frankly, she's sure to prefer you participate in some way, even if it's not exactly all on her own terms.
No matter how nervous or squeamish you feel anticipating the birth, the reality is that expectant dads rarely fall apart once they're actually in the delivery room. Supporting your wife and wanting to be the first one to greet your newborn can keep you surprisingly calm and focused.
Get additional support.
If you're worried about losing it, or just not being an effective enough labor coach, hire a doula or enlist one of your wife's sisters or friends to be in the delivery room with you.
Talk to other dads.
Spend some time talking with other men who may have been through something similar. They'll probably understand your feelings, and might have some advice to offer about how they coped with their own nervousness, or overcame their feelings of helplessness in the face of their loved one's pain. It may also be reassuring to know that you're not alone. Most will tell you that being there was a roller coaster ride of conflicting emotional states, including exhaustion, exhilaration, amazement, boredom, fear, annoyance, panic and zen-like calm. And just about all the dads will tell you that they wouldn't have missed it for the world.
If at all possible, tough it out.
No matter how eloquently you explain your reason, missing the birth is probably going to hurt your partner. So if you're not absolutely at the panic stage, consider being there for the birth for no other reason than that it'll probably make your partner happy.