Tissues are the new frontier of discoveries in immunology. Cells of the immune system are an integral part of tissue physiology and immunity. Determining how immune cells inhabit, housekeep, and defend gut, lung, brain, liver, uterus, and other organs helps revealing the intimate details of tissue physiology and may offer new therapeutic targets to treat pathologies. The uterine microenvironment modulates the development and function of innate lymphoid cells [ILC, largely represented by natural killer (NK) cells], macrophages, T cells, and dendritic cells. These immune cells, in turn, contribute to tissue homeostasis. Regulated by ovarian hormones, the human uterine mucosa (endometrium) undergoes ~400 monthly cycles of breakdown and regeneration from menarche to menopause, with its fibroblasts, glands, blood vessels, and immune cells remodeling the tissue into the transient decidua. Even more transformative changes occur upon blastocyst implantation. Before the placenta is formed, the endometrial glands feed the embryo by histiotrophic nutrition while the uterine spiral arteries are stripped of their endothelial layer and smooth muscle actin. This arterial remodeling is carried out by invading fetal trophoblast and maternal immune cells, chiefly uterine NK (uNK) cells, which also assist fetal growth. The transformed arteries no longer respond to maternal stimuli and meet the increasing demands of the growing fetus. This review focuses on how the everchanging uterine microenvironment affects uNK cells and how uNK cells regulate homeostasis of the decidua, placenta development, and fetal growth. Determining these pathways will help understand the causes of major pregnancy complications.

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Background Prior studies have demonstrated an increased stillbirth rate. It was suggested that the COVID-19 pandemic may have impacted on attendances for reduced fetal movements. Thus, we sought to ascertain the impact of the pandemic on attendances for reduced fetal movements (RFM) in our unit, ultrasound provision for reduced fetal movements, and the stillbirth rate. Methods This was a single site retrospective cohort study involving all women complaining of a 1st episode of reduced fetal movements between 01/03/2020-30/04/2020 (COVID) to 01/03/2019-30/04/2019 (Pre-COVID). Data were retrieved from computerised hospital records and statistical analyses were performed using GraphPad Prism and SPSS. Results 22% (179/810) of women presented with a 1st episode of reduced fetal movements Pre-COVID compared to 18% (145/803) during COVID (p = 0.047). Primiparous women were significantly over-represented in this population with a 1.4-fold increase in attendances during COVID (67% vs 48%, p = 0.0005). Neither the total stillbirth rate nor the stillbirth rate amongst women who presented with reduced fetal movements changed during COVID. Ultrasound provision was not impacted by COVID with 95% of the scans performed according to local guidelines, compared to Pre-COVID (74%, p = 0.0001). Conclusions There is a significant decrease in 1st attendances for reduced fetal movements during COVID-19 pandemic. Primiparous women were 1.4 times more likely to attend with RFM. Women should be reassured that COVID-19 has not resulted in a decreased provision of care for RFM, and has not impacted on the stillbirth rate.