In women who underwent fresh embryo transfer cycles, twin pregnancy were associated with a reduced female age and male age, a higher rate of lower BMI, a higher number of oocytes, a thicker endometrium, a reduced years of infertity, a higher level of progesterone, a higher rate of endometriosis, GnRH agonist use, two good-quality embryos for transfer, high education, and high ovarian response (P < 0.001, P < 0.01, P < 0.01, P < 0.01, P < 0.01, P < 0.001, P < 0.01, P < 0.01, P < 0.01, P < 0.01, P < 0.01, P < 0.01) (Table 1). Here, GNRH1 is linked to endometriosis.