(2011) performed a retrospective cohort study of all deliveries at the Mayo Clinic from 1998 to 2009, 26,314 deliveries were followed. 167 women regularly used prescription narcotics throughout their pregnancy. They said they found narcotics use in 7.5 percent of all pregnancies, which increased over the period studied. They discussed the symptoms of neonatal abstinence syndrome that appear 1 to 10 days after delivery. They indicated that the infants with the most severe withdrawal were those exposed to methadone. Kellogg et al. (2011) discussed the increasing prevalence of narcotic use during pregnancy and the increase in NAS. The study showed that NAS is rare: Only 5.6 percent of infants born to the 167 women were diagnosed with NAS due to prescription narcotics. They advised doctors to consider whether the benefits outweigh the risks when prescribing narcotics to pregnant women.