Subjects were divided into assisted reproductive treatment (ART) group (N = 167) and spontaneous pregnancy (SP) group (N = 95). Then further divided into the subgroups according to the age (< 35 years and ≥ 35 years), fertilization method (in vitro fertilization, IVF and intracytoplasmic sperm injection, ICSI), types of embryo transfer (fresh embryo transfer and frozen embryo transfer) and stages of embryo transfer (cleavage embryo transfer and blastocyst transfer).
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