They reduce pregnancy because the delay the uterus releasing the egg, and delay ovulation. Less than 1% of women who use IUDs pregnant, although pregnancies after the emergency contraceptive pill are not as rare.
The immediate physical effects of abuse or neglect (bruises or cuts) can be relatively minor or severe (broken bones, bleeding or even death). In some cases the physical effects are temporary, but it is not necessary to reduce the pain and pain caused by a child.