Statistical analysis
Statistical analyses were performed using SAS software package version
9.4. Standard descriptive statistics were used to describe the study
population: continuous variables were described using mean and standard
deviation (SD), whereas categorical variables were presented as
percentages. Proportions of sociodemographic and reproductive health
factors in Groups 1 and 2, as well as the variation between women who
conceived and those who did not conceive within the last 12 months, were
calculated using Pearson’s chi-square test and Fisher’s exact test, as
appropriate.
In our study, TTP represented the elapsed time from attempting to
conceive to confirmation of pregnancy among women who conceived within
the last 12 months (retrospective design) or the time from attempting to
conceive to the investigation time among women who had not still
conceived (cross-sectional design). To estimate a TTP distribution, we
used Kaplan–Meier curves for both populations, which could provide an
estimation of TTP and allow censors for reason other than conception. To
set up a better model, we censored observations longer than 120 months
because of the small numbers as 95% extreme values. Cox regressions
were subsequently applied to estimate the fecundability odds ratios
(FORs) associated with sociodemographic and reproductive factors for TTP
by both unadjusted and adjusted models. FORs estimate the odds of
becoming pregnant in a woman’s current cycle, conditional on not being
pregnant in the previous cycle.(8) An FOR <1 indicates reduced
fecundity or, equivalently, a longer TTP, whereas an FOR >1
represents a shorter TTP. We used the expected low-risk group in a
regression model as the reference group according to previous
literatures or clinical consensus, and results were presented with odds
ratios and 95% confidence intervals (CI) around the point estimate.
Models
were first run to estimate the crude FORs, and multivariable models were
then fitted to adjust for potential risk factors for fecundability.
Adjustments included age, ethnicity, educational level, employment
status, history of toxic exposure, smoking and drinking status of
couples, frequency of physical exercise in women, annual household
income of the family, duration of cohabitation (years), menstrual cycle
interval, history of abnormal pregnancy, and parity. Likelihood ratio
tests were used to compare the fit of models containing different
variables to test for departures from a linear trend in ordered
categorical variables. Differences with a P value of
<.05 were considered statistically significant.