[US Standard Certificate Of Birth Field 37]. Describes whether and how much the infant's mother smoked before and/or during pregnancy. The US Standard Certificate Of Birth Field 37 lists four time periods: Three months before pregnancy; first three months of pregnancy; second three months of pregnancy; and third trimester of pregnancy. It then asks for the average number of cigarettes OR packs of cigarettes smoked per day for each of these time periods. This class is modeled such that there is one instance per time period, and therefore will have up to (exactly?) four instances.