Correct answer is 3rd trimester: ➡️1st trimester villi – loose, open stroma; thick cytotrophoblast + syncytiotrophoblast; few central capillaries with nucleated fetal RBCs ➡️2nd trimester villi – denser stroma; syncytiotrophoblast + thinning/patchy cytotrophoblast; more peripheral capillary loops with mostly anucleate RBCs ➡️3rd trimester villi – dense, fibrous stroma; syncytiotrophoblast only (very thin); abundant capillary loops immediately beneath syncytium; prominent syncytial knots; fibrin deposition on villous surface; well-developed vasculo-syncytial membranes.