Eligibility and measurement of TTP
Eligibility screening of the population, selection of the target analysis set, and extraction of data on TTP are presented in Figure 1. Among 18,571 participants who responded, women aged <20 or >49 years, extremes outside the 0.5–99.5th percentile of continuous variables (BMI of women and men, age at menarche, duration of menstruation, pregnancy count, and number of uterine curettage), and couples who had separated for longer than 3 months in the past year were excluded from our study. Women who had not become pregnant within the last 12 months but had been attempting to conceive for <12 months were considered as logical error and excluded as well. Consequently, 17,275 participants who had not used contraception and had not separated from their spouse for longer than 3 months in the last year were defined as women “at risk for pregnancy” (Figure 1A). In this population, women were asked about their pregnancy status during the last year; those who had spontaneous pregnancy in the last year were subsequently asked, “How long have you been trying to conceive with this pregnancy?” Responses to these questions were used to determine their duration of current pregnancy attempt in a retrospective design among pregnant women (Group 1, fertile group). Women who did not conceive in the last year and had intercourse at <1/month were excluded. Subsequently, TTP was identified for the remaining participants via a series of questions: They were first asked about their willingness to become pregnant. For women who were attempting to conceive at the time of the investigation, the question “How long have you been trying to become pregnant?” was asked to obtain their TTP for estimation with the current-duration approach (Group 2, subfertile group). Women who did not want to become pregnant at the time of the investigation were excluded from our analysis for TTP (Figure 1B).