Embryo Implantation Female 46 Pregnancy 42 female 38 nidation 30 priority journal 30 Humans 28 controlled study 28 human 28 article 26 Animals 24 nonhuman 24 protein expression 24 pregnancy 20 Trophoblasts 18 trophoblast 18 metabolism 16 Immune Tolerance 14 Vasoactive Intestinal Peptide 14 animal experiment 14 in vitro study 14 mouse 14 vasoactive intestinal polypeptide 14 Mice 12 animal tissue 12 human cell 12 Male 10 T-Lymphocytes, Regulatory 10 progesterone 10 CD4+ T lymphocyte 8