Outcomes
Only the pregnancy outcomes from the first FET cycle were recorded in this study, and were followed up until June 2019. The primary outcome was the ongoing pregnancy rate and the secondary outcomes included incidence of premature LH surge (LH ≥10 IU/l), clinical pregnancy, miscarriage, multiple pregnancy, ectopic pregnancy and implantation rates. Clinical pregnancy was defined as the presence of at least one gestational sac on ultrasound at 6 weeks. Ongoing pregnancy was the presence of at least one fetus with fetal pulsation on ultrasound beyond 10 weeks. Fertilization rate was the percentage of zygotes with two visible pronuclei among inseminated (IVF) or matured (ICSI) oocytes. Miscarriage rate was defined as the number of miscarriages before 20 weeks divided by the number of women with positive pregnancy test. Multiple pregnancy was defined as a pregnancy with more than one gestational sac detected on ultrasound at 6 weeks. The implantation rate was calculated as the number of gestational sacs seen on scanning divided by the number of embryos replaced.