Coelomic fluid (n = 32), amniotic fluid (n = 26) and placental tissue were obtained from 32 women undergoing termination of pregnancy at 7–11 weeks of gestation. Fluorescence in situ hybridisation (FISH) was performed to determine fetal sex using a heterochromatic Y probe and an alpha satellite repeat probe for chromosome X. In each case there was concordance in the fetal sex from the three compartments. However, no result could be obtained from 4 of the 32 coelomic fluids and 4 of the 26 amniotic fluid samples. The hybridisation efficiency was significantly lower in coelomic and amniotic fluid compared with placental tissu