Last year, when Mariah Leach was pregnant with her son, her rheumatoid arthritis flared up again. She had stopped medication during pregnancy, but when the autoimmune disease, which causes painful and damaging inflammation in joints and elsewhere, resurged, she and her doctors made the difficult decision for her to restart during her third trimester, based on a very limited set of data that suggested a medication she had used previously could be taken safely during pregnancy.