Study design and population
The current project is a large-scale population-based cross-sectional study conducted in northern and eastern China from 2010 to 2011. Details about sampling, eligibility, and investigation have been introduced elsewhere.(3) Briefly, our study utilized a multistage stratified cluster sampling strategy to recruit participants from each stratum by district, province/municipality, town/township, and village/street order (Figure S1). Couples were then approached by professional investigators with the assistance from local officials and communities. Ultimately, 25,270 couples were approached from 8 provinces/municipalities/cities (including Zhejiang, Anhui, Fujian, Shandong, Beijing, Hebei, Heilongjiang, and Nei Mongol) for their sexual and reproductive health information.Among these couples, 18,571 were interviewed, with a response rate of 74% (18,571/25,270).