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.
Avoidance. Of course the best case scenario is that you avoid carbon monoxide poisoning entirely. The best way to do this is with a carbon monoxide detector which you can get from most supermarkets and general stores. By testing each room over night you can identify a possible problem and thus take action to track the source of the problem and have it repaired.