To investigate chorionic villous vasculogenesis(maturation)and development of the vasculosyncytial membrane(margination)using CD34immunohistochemistry.C ase -control study.Microscopic analysis of first trimester chori-onic villi.Twelve patients with anembryonic pregnancies,12with embryonic death,and 12with t erminated normal pregnancies.Quantitative analysis of chorionic villi blinded to group and gestational age using CD34immunohisto-chemistry.Vascular parameters(mean functional vascular area,vessels with a lumen,and heman giogenetic cords,peripherally or centrally located).Terminated normalpregnancies show significantly more vessels per chorionic villus(maturation)(mean ±SEM)in comparison with embryonic deaths and anembryonic pr egnancies(5.3±0.3vs.1.4±0.2and 0.7±0.1),located mainly peripherally(margination)(3.0±0.2vs.0.9±0.2and 0.2±0.0).Anembryonic pregnancies show sign ifi-cantly more centrally located cords in comparison with embryonic deaths and termination of pregnancies(3.3±0.2vs.2.7±0.2and 1.5±0.1).A defective chori-onic villous vascularization,demo nstrating inadequate vasculogenesis and abnormal develo pment of the vascu-losyncytial membrane,is seen in pre gnancies complicated by embryonic death and is even more pr onounced in anembryonic pregnancies.Initiation of placental vasculo-genesis is a basic feature in all types of pregnancy and is subsequently modulated directly or indirectly by embryonic signaling. To investigate chorionic villous vasculogenesis (maturation) and development of the vasculosyncytial membrane (margination) using CD34 immunohistochemistry. C ase -control study. Microscopic analysis of first trimester chori-onic villi. Patients with anembrytic pregnancies, 12 with embryonic death, and 12 with t erminated normal pregnancies. Quantitative analysis of chorionic villi blinded to group and gestational age using CD34 immunohisto-chemistry. Vascular parameters (mean functional vascular area, vessels with a lumen, and heman giogenetic cords, peripherally or centrally located.) Cerminated normalpregnancies show significantly more vessels per Chorionic villus (maturation) (mean ± SEM) in comparison with embryonic deaths and anembryonic pr egnancies (5.3 ± 0.3 vs. 1.4 ± 0.2 and 0.7 ± 0.1), located mainly peripherally (3.0 ± 0.2 vs. 0.9 ± 0.2 and 0.2 ± 0.0). Anembryonic pregnancies show sign ifi-cantly more centrally located cords in comparison with embryonic deaths and termination of pregnancies ( 3.3 ± 0.2 vs. 2.7 ± 0.2 and 1.5 ± 0.1). A defective chori-onic villous vascularization, demo nstrating inadequate vasculogenesis and abnormal develoment of the vascu-losyncytial membrane, is seen in pre gnancies complicated by embryonic death and is even more pr onounced in anembryonic pregnancies. Invitations of placental vasculo-genesis is a basic feature in all types of pregnancy and is subsequently modulated directly or indirectly by embryonic signaling.