Neonatal screening for congenital adrenal hyperplasia (CAH) faces many specific challenges. It must be done using a performant analytical approach that combines sensitivity and specificity to capture the potential causes of mortality during the first week of life, such as salt-wasting and glucocorticoid deficiency. Here, we confirm that maternal inhaled corticosteroid intake during pregnancy is a possible cause of missed CAH diagnosis. Thanks to liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) analysis, we were able to quantify endogenous steroid metabolites and also detect the presence of exogenous steroids in the dried blood spot of a newborn. Adding LC-MS/MS analysis as second-tier test, especially one that includes both 17-hydroxyprogesterone and 21-deoxycortisol measurements, would probably improve CAH diagnosis. In familial neonatal screening one could also look for maternal corticosteroid therapies that are hidden to prevent false negative tests.