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To be honest, there is no consensus with regards to the exact number normal range of natural killer cells and/or its predictive value for obstetric pathology. This is stated by the Human Fertilization and Embryology Authority which is actually a UK government fertility regulator. Abnormal numbers and dysfunction of the decidual natural killer population could result in pregnancy loss as it has been suggested by studies that quantify NK population changes as they are reflected in peripheral blood. Peripheral blood natural killer cells measurements may be reported as absolute numbers but more frequently as a percentage of total leukocytes. There is little consensus as to what constitutes a high peripheral blood natural killer cell number. Different studies have used cutoffs of between 12 up to 18 %.
