A study conducted by Chambers et al. in 2006 indicated that newborns whose mothers had started taking an SSRI such as Paxil after twenty weeks of becoming pregnant are six times more likely to develop PPHN than the general population. Another study by Källén et al. reported a significant relationship between the occurrence of PPHN and the use of antidepressant drugs such as Paxil during the first trimester of pregnancy. No association between the use of the SSRI drug during the third trimester of pregnancy and PPHN was found by the study conducted by Andrade et al. The FDA updated its safety warning in December 2011. As the review of more recent studies yielded additional findings that had to be taken into account, it was concluded that a final decision cannot yet be established about the relationship between the use of SSRIs and the development of PPHN.