The birth of Christ in Matthew is an interesting mix of God intervening. Mary and Joseph and engaged, but Mary becomes pregnant, and not due to Joseph. This isn't good in our more egalitarian society, but this would have been really be in patriarchal one. I was just reading last night in the 28 stories of AIDS victims book that I mentioned earlier, about a woman in Swaziland. She had only slept with one man, her husband. Despite many obstacles, she became a real leader in her community, a status few if any women reach in her country. But some 10 years into their marriage, when a blood test was taken and she was determined to have AIDS, she was immediately shunned. Rumors spread like wild-fire throughout the community. Surely when she was off on her business trips, she was sleeping around, they thought. Non one would even think to raise the question of her husband's fidelity despite the common practice in their culture of multiple wives/girlfriends. She received no intervention from God, and had to bear the shame of the disease and the implied unfaithfulness alone. She received no intervention from God.
I would be that similar thoughts were racing through people's minds about Mary. Joseph knew that he wasn't the father, so he naturally concluded that it was some other guy. He was right that it was someone else, but not right that it was someone else in the community. God intervened into the situation, and informed Joseph that God's Spirit had indeed conceived within Mary. "There's no marital unfaithfulness here. Keep her, and father the child that is develop within her."
After Jesus is born and Herod realizes that he has been tricked, he is enraged and decides to kill children 2 and under in the community. In this case, God doesn't intervene for the Bethlehem children that will be slaughtered as a result of Jesus being born there. Joseph, Mary, and Jesus escape, but the other families experience the tragic experience of having their innocent young children killed.
There all kinds of cases through Scripture and history where God intervenes, and cases where he doesn't. I wish I had a corner on the "predicting when God's going to intervene" market, but I don't. It's definitely a mystery. The only way God amongst this mystery is to recognize that death and this life are not the end. The lifespan of the children Herod killed was not their 2 short years in Bethlehem. They are undoubtedly with God now experiencing life as it was originally intended. Let us continue to plead God to intervene in this world, and at the same time recognize, that this world is not the end.