Biochemical tests have been used to monitor fetoplacental function, mainly to detect impending fetal death in utero. The specimen-sampling patterns employed have not been adequate to detect all fetal deaths, and increasingly these tests have been to detect low-birthweight babies who are at risk during the perinatal period. The purpose of this study was to investigate the three fetoplacental function tests, namely, serum total oestrogens, human placental lactogen, and cystine aminopeptidase activity under routine conditions to indicate whether these tests detected low-birthweight babies and, particularly, if they aided the clinical assessment of pregnancy. One hundred and sixty-six patients were studied; each set of results was classified as normal or abnormal and subsequently compared according to the outcome of pregnancy. In addition, a comparison was made between abnormal results where intervention was deemed necessary or unnecessary on clinical grounds. The conclusion reached were that these tests played a minor role in the assessment of pregnancy, at best being used to alert the obstetrician to an increased risk of an "abnormal birth.'