Selenomethionine Uptake Test as a Sensitive Indicator of Placental Function in the Last Trimester of Pregnancy

During the last trimester of pregnancy the demands of the human foetus for nutrients can only just be met by the normal placenta. If the placenta is damaged or poorly developed foetal growth is impaired or, in severe cases, the baby dies. It is clinically important to detect placental failure as early as possible so that the underlying cause can be treated, or if this is not possible the baby can be delivered before term. 75Se-selenomethionine is one of the amino-acids which is taken up by the placenta from the mother's blood and actively transported against a concentration gradient to the foetus. In the selenomethionine uptake test two 3-in. diameter Nal detectors in cylindrical collimators are used; one is positioned over the mother's mediastinum and the other over her uterus. The output from each detector is fed through a pulse-height analyser to a recording ratemeter. A dose of 2 μCi of selenomethionine is given intravenously to the mother, and from the recorded count-rates during the next 15 minutes the efficiency of placental amino-acid transport can be assessed. The maximum radiation dose to the mother or foetus is 20 mrad. This test has been applied without ill effects to over 500 patients in whom there were clinical grounds for suspecting placental damage. Gross placental failure and intrauterine death were invariably associated with a very low selenomethionine-uptake over the uterus, and high uterine uptakes were always associated with good placental function. Misleading results may be obtained in cases with rhesus immunization or congenital defect in the foetus, since in such cases foetal growth failure is not due to a defect of transport in the placenta. Generally the test provides a quick, simple and reliable indication of the nutritive function of the placenta, and can be safely used to select those mothers who need particularly careful clinical management. (author)