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.
By the way, Sacramento County Code Section B-8 R315.1 and R315.2 says the following regarding where carbon monoxide detectors should be installed (exactly what I said above): Outside of each separate dwelling unit sleeping area in the immediate vicinity of the bedroom(s).