OR, odds ratio; CI, confidence interval; NC, natural cycle; AC, artificial cycle.
a We did not adjust other covariates.
b Adjusted for female age at embryo transfer, infertility duration, BMI, AFC, no. of oocyte retrieved, infertility type, protocol in the fresh cycle, single blastocyst transfer, endometrial thickness on the day of P administration, triple-line endometrial pattern, type of embryo transferred, no. of transferred embryos, no. of transferrable embryos and no. of good quality embryos transferred.