OPRM1 and placental insufficiency: Some genes implicated by the top 300 SNPs from maternal GWAS in labor-initiated deliveries could be comfortably assigned to these processes: infection/inflammation (NFKB1, TLR4, IRF8, ABCA1, TREML2, MEFV, WNT5A, NFKBIA), placental insufficiency (ENG, TOR2A, IGF2, KCNQ3, GNB3, LEFTY2, ARHGEF11, WNT4, WNT5A), cervical insufficiency (MMP9, TIMP2), and hormonal imbalance (WNT4, OPRM1, SP3).