Large congenital nevi form in the womb very early in development, within the first twelve weeks of pregnancy. They are caused by a defect during embryologic development. There is no known method of prevention. They appear in either sex, in all races, and on any areas of the skin.
I recommend that you have a carbon monoxide detector within 15 feet of each bedroom door, 10 feet is even better if you can afford more detectors. If two bedroom doors are 30 feet apart or less, then place one in the middle. If however they are more than 30 feet apart you will need more than one detector.
Proper placement of a carbon monoxide (CO) detector is important. If you are installing only one carbon monoxide detector, the Consumer Product Safety Commission (CPSC) recommends it be located near the sleeping area, where it can wake you if you are asleep. Additional detectors on every level and in every bedroom of a home provides extra protection against carbon monoxide poisoning.