Those who have delved deeper into the subject know that the opening in the human pelvis through which the baby must pass is limited in size by our upright posture .

• 对这个问题下过一些工夫的人知道：人类的骨盆开口，是胎儿出生必经的出口，但是我们的直立体态，却使那个出口的尺寸受到极大的限制。