Rachael - They take a biopsy from your uterine wall to test for NK cells. Some FS only take a BT but that gives alot of false results. The biopsy should be done around CD 24 to CD 26 but must be done when you aren't in an active cycle. During this time your aren't cycling would be perfect time to have the test but it will all depend on your FS. There are alot that don't believe NK cells make a difference but I have a 10 month old who wouldn't be here if I hadn't of been tested and treated.