Nonimmune hydrops (NIH) and severe symmetrical intrauterine growth restriction (IUGR) represent two obstetrical circumstances that occur with relative frequency yet often escape adequate etiology assessment and diagnosis prior to and after birth. Both of these conditions have high perinatal and neonatal mortality rates. While antepartum ultrasound and amniotic fluid evaluations of fetal karyotype and viral DNA studies diagnose some etiologies for NIH and severe symmetrical IUGR, large percentages of NIH (30-40%) and IUGR (20-30%) cases are attributed to idiopathic causes. Because of this uncertainty in diagnosis, many cases are subjected to prolonged antepartum hospitalization with intensive fetal monitoring and urgent delivery by Cesarean section for non-reassuring fetal status, only to succumb to a neonatal demise in the nursery. A small percentage (1-2%) of these cases are found to be due to inborn errors of metabolism by neonatal and/or postmortem evaluation; however, when combined together, greater than 50% of NIH and IUGR cases have no identifiable etiology. Therefore, any new test that may make a diagnosis for NIH or severe symmetrical IUGR prior to delivery would be extremely important to the management of both the mother and the infant. Tandem mass spectrometry for inborn errors of metabolism may represent a new tool for assessing, identifying, and treating currently unexplained cases of NIH and severe symmetrical IUGR.