NK cell testing is usually performed after recurrent miscarriage or repeated IVF failure.  If you are unsure of the advice you have been given by your doctor then I would suggest that you to seek a second opinion.  At the age of 40 your chance of success with IVF is low as the quality of your eggs decreases as you get older. Before we could offer any advice about further treatment we would need to see all the details of your recent cycle