Like all drugs that enter the bloodstream, nitrous oxide is distributed throughout the body. It also passes very easily across the placenta and is distributed likewise throughout the baby's body. Unlike opioid drugs, however, nitrous oxide is excreted from the body very quickly - and entirely - by the lungs. It does not have to be broken down (or metabolised) first by the liver and so there are no 'by-products'. This rapid elimination of nitrous oxide also applies in the case of the baby: within five minutes of birth it cannot be detected in the baby's breath at all. Because of its rapid elimination, it doesn't really matter how long nitrous oxide is used; the gas does not 'build up' or accumulate to any degree whether it is used for five minutes or five hours.