A head-first birth is safest. Breech births are riskier for mother and child.

In the last weeks of pregnancy, the baby typically moves into a vertex or head-down position, where he remains until birth. In the time of Hippocrates, the ancient Greek father of medicine, physicians believed that the fetus assumed this position so that he could kick off from the top of the uterus and push himself through the birth canal. Although the explanation is wrong, the fact is that the head-first delivery is best.

As labor continues and the baby descends into the birth canal, he turns his head. "The baby goes from typically looking sideways, toward one hip or the other, and rotates so that the back of the head is up and the baby is facing toward the mother's back," explains Dr. Copel.

That backward-facing head offers the smallest diameter for slipping through the birth canal. When the baby emerges facing upward, or "sunny side up," for example, he says, the fit is tighter because the distance from the forehead to the chin is greater than the distance from the crown of the head to the back of the neck.