The baby may be stillborn or may die shortly after birth from the effects of anemia. This used to be a serious problem in a significant number of families. But a painstaking program of detection, prevention, and treatment has now made it manageable, at least in developed countries like the United States. The collar of muscle and connective tissue that forms the passage from the uterus usually remains firmly closed until the end of pregnancy, when it opens up to allow delivery. But some women have a weak, or incompetent, cervix, which opens in midpregnancy and causes a miscarriage or premature delivery.
Incompetence is apparently a physical abnormality of the cervix.
In many cases the cause is unknown. The risk for incompetence also rises after the cervix is damaged by surgery or by a tear during a previous delivery. It describes a placenta that is in the way of birth - partly or entirely covering the entrance to the cervix at the bottom of the uterus . Why a fertilized egg becomes implanted so low on the wall of the uterus is generally unknown. But as pregnancy approaches full term and the cervix begins to open, a placenta previa may tear loose and bleed, sometimes severely.