Study population
The study was approved of the Ethic Committee of The First Affiliated Hospital of Xi’an Jiaotong University according to the declaration of Helsinki. From January 2008 to December 2018, a total of 5680 couples who had two or more spontaneous miscarriage before the 20th week of pregnancy came to our reproductive center for outpatient treatment. The included patients must provide objective evidence of past birth history, including HCG testing, or the histology report after curettage and evacuation of uterine or the gestational sac under ultrasound, all the clinical data bring to the study was carefully recorded and checked.