They are both correct. "Sigmoid" alludes to the "C" or "S" shape of these valves, also adquately described as "semilunar".

The sigmoid or semilunar valves are the ones at the root of the aorta and pulmonary artery, and prevent the blood flow from returning to the heart once it's reached the great vessels.

Below I'm including a couple of links to medical articles that name these valves with either name. However, in both American and British English, "semilunar" is much more frequently used to name these valves than "sigmoid".

Buon lavoro
Elena
(MD in the US, have worked in a hospital in Bologna)