In the window of implantation, progesterone plays an important role. Sufficient serum progesterone is basic for ongoing pregnancy. Vaginal progesterone is more and more widely used in ART. As it has no hepatic first pass effect. What is the optimal serum level for pregnancy when use vaginal progesterone is not known yet? Hormone replacement therapy- FET is the optimal strategy to explore this question. There are some retrospective studies showed that the serum progesterone level on embryo transfer day (D3 or D5) or pregnancy test day (D14) lower than 10-11ng/ml is significantly associated with ongoing pregnancy rate in HRT-FET cycles. This prospective study is designed to compare the ongoing pregnancy rate between different serum progesterone levels on D3 and to explore the intervention of additional progesterone supplement since D3 is helpful in HRT-FET cycles.

Inclusion Criteria:

1. HRT-FET cycles，including GnRHa-HRT-FET
2. Age<41 years old
3. BMI<30kg/m2
4. TSH and PRL normal
5. Endometrium thickness ≥8mm on D0 6） embryo transfer completed with at least 1 top-quality embryo (7-9 cell Grade 1 or D5 beyond grade 3 ) 7） Scandalized luteal phase support： transvaginal progesterone 0.2 tid with or without oral progesterone 10mg bid 8） included once for every patient

Exclusion Criteria:

1. history of Moderate and Severe uterine adhesion；
2. presence of hydrosapinx diameter >2cm
3. Endometrosis at stage III-IV
4. Recurrent implantation failure (>3 times of embryo transfer cycle)