Data collection
Based on the results of a pilot study, a standardized and structured questionnaire was designed after discussion with gynecologists and epidemiologists. Face-to-face interviews were conducted by trained interviewers consisting of local community leaders and healthcare workers. In the investigation process, women were interviewed to recall information about their partners and themselves. Household interviews were carried out in private to assure the confidentiality of the information obtained.
Data on general demographic and socioeconomic status, physical measurements, lifestyle habits, and toxic exposure for both women and their partners were collected, including the region, age of couples (represented as the current age of the couple minus the number of years attempting to conceive and classified as 20–24, 25–29, 30–34, 35–39, and ≥40 years), ethnicity (Han ethnic group and minorities [including Mongol nationality, Hui nationality, and Zang nationality]), body mass index (BMI; in kg/m2) of couples (<18.50, 18.50–23.99, 24.00–27.99, and ≥28.00 kg/m2), educational level (primary school and below, middle school, high school, college and beyond), current occupation (employment [included clerk, professional worker, service worker, agricultural and related worker, factory worker], unemployment [referred to being a housewife, job loss, and retirement], and others or not stated), history of toxic exposure (“yes” [restricted to radioactive and toxic substances] or “no”), smoking (“yes” [defined as currently smoking at least once a week] or “no” (included those who never smoked and used to smoke]), drinking (“yes” [defined as currently drinking at least once a week] or “no” (included those who never drank and used to drink]), physical exercise (occasionally, 2–3 times a month, and at least once a week), and annual household income (low [<10,000 renminbi (RMB)], middle [10,000–19,999 RMB], or high [≥20,000 RMB]). Additionally, reproductive health, marriage, and bearing status were asked, including the duration of cohabitation (represented as the original duration of cohabitation minus the number of years attempting to conceive), frequency of intercourse (1/day, 1/2-5 days, 1/week, 1/10-15 days and 1/month), menstrual cycle interval (≤23, 24–35 and >35 days), history of abnormal pregnancy (“yes” [included a history of stillbirth, induced labor, spontaneous abortion, and ectopic pregnancy] or “no”), and parity (nullipara and multipara).