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.
Any standard carbon dioxide detector with a battery can be used. Place the carbon monoxide detector in the vehicle, close the windows and vents, start the vehicle and immediately exit. After 10 minutes, shut down the vehicle, and check the detector's readings to determine if there is a carbon monoxide leak in the car.