We present the case of a 39-year-old female patient, 59 kg and 160 cm tall, with a history of four spontaneous abortions of seven to ten weeks with a live embryo. The sterility studies she had undergone were within normal limits, except for a diagnosis of heterozygous Factor V Leiden6. This alteration results in a variant of human coagulation factor V that frequently causes an inherited hypercoagulability disorder, most commonly among Eurasians. In these cases, the factor V Leiden variant cannot be inactivated by activated protein C. In pregnant women with this abnormality, prophylactic administration of low molecular weight heparin (LMWH) at therapeutic doses adjusted per kg of body weight is recommended as soon as gestation is diagnosed and up to 6-8 weeks postpartum. It should be noted that in the last gestation the patient experienced a miscarriage despite being treated with LMWH, so the haematological origin of these miscarriages is ruled out.
With the previous obstetric history (G4P0A4), the patient began a new pregnancy with a therapeutic regimen of LMWH enoxaparin 40 mg/24 h, as soon as she was aware of this. At five weeks the patient underwent an immunological study. The results showed NK cell values above normal, with 18% of peripheral NK cells and 95% of them with CD56+CD16 expression (17% of the lymphocytes). According to the algorithm proposed by Ramos-Medina3 et al. for the patient's age, these values imply a high risk of presenting an immunological alteration associated with reproductive failure. Based on the study carried out, they conclude that: a percentage of NK cells > 18% is the best variable for discriminating women with reproductive failure, and for women over 35 years of age, CD56+CD16 cell values above 13% define a special subgroup of patients with a high risk of pregnancy loss (100%).
Off-label use of this drug is requested. After authorisation by the centre's management and the signature of the pregnant woman on the informed consent form, the administration of the IGs regimen described in the literature was started. It starts with GIs 400 mg/kg body weight, every four weeks, from the knowledge of week 4 to week 13, once a month; then 200 mg/kg monthly until week 36, since it has been shown that GIs cross the placenta more intensively after the third trimester. After two doses of IGs, a new immunological study is requested, showing a reduction of NK cells to 8% with 90% CD56+CD16 markers (7.2% of lymphocytes). Figure 1.

Treatment was administered according to schedule with no incidents or relevant adverse effects with respect to the established IG regimen.
At week 40 + 1 the pregnant woman had a live birth by elective caesarean section. She was switched to a thromboprophylactic regimen of LMWH the day before the operation and discontinued the same day. The therapeutic dose is restarted at 12-24 hours and continued for two months.