(2011) performed a retrospective cohort study of all births at the Mayo Clinic from 1998 to 2009, 26,314 births were followed. 167 women regularly used prescribed narcotics during pregnancy. They said they found narcotic use in 7.5 percent of all pregnancies, an increase over the time period studied. They discussed that neonatal abstinence syndrome symptoms appear 1 to 10 days after delivery. They reported that the children with the most severe withdrawal were the children 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% of children born to 167 women were diagnosed with NAS due to prescription narcotics. They advised doctors to check whether the benefits outweigh the risks when prescribing narcotics to pregnant women