Uterine Natural Killer (uNK) Cell Expression in Patients With Repeated ICSI Failures: A Multicenter Cohort Study

For occurrence of the "window of implantation", the endometrium must proliferate, increase in thickness, then, after ovulation, adequately respond to progesterone, and become receptive. Ultrasound examination of the thickness and appearance of the endometrium is an easily performed means of assessing morphological changes occurring in the endometrium during the follicular phase, and is thus used as a measure to predict successful implantation. Indeed, several studies have reported a strong association between endometrial thickness and successful implantation. However, others failed to confirm such an association. The minimal adequate endometrial thickness for successful implantation, as measured in the late proliferative phase, varies between studies, with a range of 6-8 mm. However, although rare, some investigators have reported successful implantation in an endometrium of no more than 5 mm thickness.

Uterine NK cell testing can only be done via an endometrial biopsy. Uterine natural killer cells never come into direct contact with the fetal cells-they only have direct access to placental cells.The biopsy is performed on day 21 of your menstrual cycle, when the NK cell count is at its highest. The procedure is done at outpatient clinics and results take about two weeks to receive. If your NK cell activity reading is greater than 14 % on day 21 biopsy, then diagnosed with high levels of NK cell activity.