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We report a MEMS contour-mode film bulk acoustic resonator (C-FBAR) to monitor whole blood coagulation in-vitro in real time. The C-FBAR has a suspended ring of piezoelectric aluminum nitride excited in the radial-extensional mode. It operates at its resonant frequency of 150 MHz and possesses a quality factor (Q) of 140 in water. The C-FBAR is characterized using aqueous glycerine solutions showing that it accurately measures the viscosity in the range of 1 to 10 centipoises. The C-FBAR, then, is used to monitor blood coagulation in-vitro in real time. The resonant frequency decreases as viscosity of the blood increases during the fibrin creation after the coagulation cascade. The coagulation time and the start/end of the fibrin creation are determined, suggesting the C-FBAR as a potentially low-cost, portable tool for hemostasis diagnostics.