Definition of pregnancy
A serum β-hCG assay 11-14 days after ET. Clinical pregnancy was defined as the presence of a gestational sac on transvaginal ultrasound.
Clinical miscarriage was defined if the pregnancy terminated before 12 weeks of gestational age. Implantation rate was defined as the number of intrauterine sacs divided by the number of embryos transferred.
Ongoing pregnancy was defined as gestations that reached 20 weeks or more. Live birth was defined as give birth to an infant ≥24 weeks’ gestation.