the innate immune system, are cytotoxic and responsible for attacking pathogens and infected cells. However, the number and type of receptors the uNK cells contain during a healthy pregnancy differs compared to an abnormal pregnancy. In the first trimester of pregnancy, uNK cells are among the most abundant leukocytes present, but the number of uNK cells present slowly declines up until term. Despite the fetus containing foreign paternal antigens, uNK cells do not recognize it as "non-self". Therefore, the cytotoxic effects of the uNK cells do not target the developing fetus. It has even been proposed that uNK contributes to the protection of extravillous trophoblast (EVT), important cells that contribute to the growth and development of a fetus. uNK cells secrete transforming growth factor-β (TGF-β) which is believed to have an immunosuppressive effect through modulation of leukocyte response to trophoblasts. Killer immunoglobulin-like receptors (KIRs) and human leukocyte antigen (HLA) KIRs are expressed by the uNK cells of the mother. Both polymorphic maternal KIRs and fetal HLA-C molecules are variable and specific to a particular pregnancy. In any pregnancy, the maternal KIR genotype could be AA (no activating KIRs), AB, or BB (1–10 activating KIRs) and the HLA-C ligands for KIRs are divided into two groups: HLA-C1 and HLA-C2. Studies have shown that there is a bad compatibility between specifically maternal KIR AA and fetal HLA-C2 which leads to recurrent miscarriage, preeclampsia and implantation failures. In assisted reproduction, these new insights could have an impact on the selection of single embryo transfer, oocyte, and/or sperm donor selection according to KIRs and HLA in patients with recurrent miscarriages. Medication exposure during pregnancy Pharmacokinetics Pregnant-related anatomical and physiological changes affect pharmacokinetics (absorption, distribution, metabolism, and excretion) of many drugs, which may require drug regimen adjustment. Gastrointestinal motility is affected by delayed gastric emptying and increase gastric pH during pregnancy, which may alter drug absorption. Changes in body composition during pregnancy may change drugs volume of distribution due to increased body weight and fat, increased total plasma volume, and decreased albumin. For drugs susceptible to hepatic elimination are influenced by increased production of estrogen and progesterone. In addition, change in hepatic enzyme activity may increase or decrease drug metabolism based on drug composition, however most hepatic enzymes increase both metabolism and elimination during pregnancy. Also, pregnancy increase glomerular filtration, renal plasma flow, and the activity of transporters, which may require increased drug dosage. FDA