Sample characteristics
Among 17,275 women deemed at risk for pregnancy, 6,981 (40.4%) had spontaneous pregnancy in the last year, which composed Group 1 at the time of the investigation (5,422/6,981, 77.7%) after excluding missing values for TTP (1,543/6,981, 22.1%) and couples with TTP >120 months (16/6,981, 0.2%) (fertile group ). Except for 234 missing values (234/17,275, 1.4%), 10,060 (58.2%) out of 17,275 women did not conceive in the last year. When asked if they were willing to become pregnant, 3,328 couples answered “yes”; of these 3,328 couples, 2,467 (74.1%) were subsequently selected as Group 2 after excluding missing values (497/3,328, 14.9%) and couples with TTP >120 months (364/3,328, 10.9%), which could be regarded as the population with a subfertile status at the time of the investigation (subfertile group ). After excluding 6,040 (35.0%) out of 17,275 women who were not willing to become pregnant, a total of 7,889 (45.7%) women conceived or had been attempting to become pregnant in the last year (Figure 1B).
As presented in Table 1, couples in Group 2 were older than those in Group 1 (women: 26.4 ± 4.9 [mean ± SD] years in Group 2 vs. 25.6 ± 4.3 years in Group 1; men: 28.5 ± 5.2 years in Group 2 vs. 27.6 ± 4.8 years in Group 1). Group 2 had a higher proportion of currently overweight men (23.0 ± 2.2 kg/m2 in Group 2 vs. 22.7 ± 2.2 kg/m2 in Group 1). Couples in Group 1 were more educated than those in Group 2, and couples in Group 2 had higher annual income. More subfertile couples were exposed to toxic substances (women: 3.8% in Group 2 vs. 1.4% in Group 1; men: 5.7% in Group 2 vs. 3.3% in Group 1). Furthermore, male spouses were more likely to smoke (45.7% in Group 2 vs. 42.7% in Group 1) in the subfertile group. Subfertile women tended to have longer duration of cohabitation (6.1 ± 4.8 years in Group 2 vs. 3.7 ± 3.7 years in Group 1), and 8.9% of women in the subfertile group had a menstrual cycle interval longer than 35 days (vs. 2.9% of women in the fertile group). The proportion of multiparous women and women with a positive history of abnormal pregnancies was higher in the fertile group (33.1% in Group 1 vs. 29.0% in Group 2 and 89.5% in Group 1 vs. 85.9% in Group 2, respectively).