Observational parameters and outcome variables
Baseline parameters included age, BMI, duration of infertility, primary infertility, antral follicle count (AFC), basal follicle stimulating hormone (FSH), anti-Müllerian hormone (AMH), the mean diameter of initial uterus, history of dysmenorrhea and IVF-related parameters. The mean diameter of uterus was calculated as the average of long and wide diameter in longitudinal section measured by TVS on day 2-6 of the menstrual cycle. According to the verbal multidimensional scoring system(14), dysmenorrhea was classified into none, mild, moderate and severe. The primary pregnancy outcome was CPR, and the secondary pregnancy outcomes included implantation rate (IR), biochemical pregnancy rate (BPR), ectopic pregnancy rate (EPR), miscarriage rate (MR), early MR, late MR and LBR. Serum β-hCG level was examined 14 days after ET. Biochemical pregnancy loss was defined as elevated serum β-hCG level in the absence of invisible intra-uterine and extra-uterine gestational sac. Clinical pregnancy was defined as the presence of intra-uterine pregnancy. Ectopic pregnancy was defined as the presence of gestational sac or mass outside uterine cavity. Miscarriage was defined as pregnancy loss before 28 gestational weeks. Clinical pregnancy loss ≤12 gestational weeks was early miscarriage, vice versa, pregnancy loss >12 gestational weeks was late miscarriage. IR was defined as the ratio of total numbers of gestational sac confirmed by TVS to total numbers of transferred embryos.