SHBG and metabolic syndrome: PCOS: polycystic ovarian syndrome; GDM: gestational diabetes mellitus; PIH: pregnancy-induced hypertension; PCO: polycystic ovary; IVF: in vitro fertilization; ICSI: intracytoplasmic sperm injection; SHBG: sex hormone-binding globulin; MetS: metabolic syndrome; WHO: World Health Organization; T2DM: type 2 diabetes mellitus; PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses; ART: assisted reproductive technology; HbA1c: glycosylated hemoglobin.