The foetal blood is separated from syncytiotrophoblast with all except:
The maternal blood in the lacuna is never in direct contact with fetal blood. They are separated by:

Syncytiotrophoblast
Cytotrophoblast
Basement membrane
Mesoderm
Endothelium of fetal capillaries

Together called as placental barrier or membrane (0.025 mm)
Fetal blood capillary membrane
Mesenchyme of intervillous blood space
Cytotrophoblast
Decidua parietalis