Pregnant 12 years after vasectomy

My fiancé has been medically sterile for 12 years due to vasectomy, but I'm pregnant. An mri showed recanalization, but his doctors say any sperm that may get through are definitely deformed. In a sample, many showed monosomy and trisomy. They say any pregnancy would not be viable and more than likely it's a non-medical cause: aka they accuse me of infidelity, which is impossible. This is my first pregnancy and I'm now 20 weeks. I've had every test under the sun. Defying the odds, my little girl is completely healthy. The only result I'm missing is the amnio paternity test. I know it's his baby and she's a miracle. I haven't told my family yet, just in case something was seriously wrong and I would have to make the decision to end it. But I know in my heart I could never do that. Now my fiancé is all of a sudden saying I should reconsider. I don't understand. After all we've been through? I'm 25 and he's 40. We can do this together, right? I may not be ready, but we're responsible adults. 
Give me some advice and encouragement.