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).