TABLE 2.
HRs summarizing the effect of preconception DDT and B-vitamin status on incidence of clinical pregnancy in 291 women prospectively followed from preconception to clinical pregnancy or up to 12 mo (whichever occurred first)1
| Low DDT |
High DDT |
||||||
| HR (95% CI) |
HR (95% CI) |
||||||
| Median time to CP, cycles2 | Crude | Adjusted | Median time to CP, cycles2 | Crude | Adjusted | P-effect modification | |
| Vitamin B-12, pmol/L | 0.023 | ||||||
| Normal | 2.9 | 1.00 (reference) | 1.00 (reference) | 2.9 | 1.01 (0.78, 1.31) | 1.03 (0.78, 1.36) | |
| <258 | 2.7 | 1.12 (0.72, 1.73) | 1.17 (0.74, 1.86) | 5.3 | 0.44 (0.26, 0.75) | 0.51 (0.29, 0.90) | |
| Linear vitamin B-12, /IQD | — | — | 0.80 (0.65, 0.99) | — | — | 1.23 (0.98, 1.54) | 0.0054 |
| Vitamin B-6, nmol/L | 0.973 | ||||||
| Normal | 2.6 | 1.00 (reference) | 1.00 (reference) | 2.7 | 0.96 (0.73, 1.26) | 0.91 (0.68, 1.21) | |
| <30 | 3.8 | 0.61 (0.40, 0.92) | 0.54 (0.35, 0.83) | 4.6 | 0.46 (0.31, 0.67) | 0.49 (0.33, 0.72) | |
| Linear vitamin B-6, /IQD | — | — | 1.22 (1.001, 1.49) | — | — | 1.73 (1.39, 2.16) | 0.024 |
| Folate, nmol/L | 0.993 | ||||||
| Normal | 2.7 | 1.00 (reference) | 1.00 (reference) | 3.0 | 0.84 (0.64, 1.09) | 0.88 (0.67, 1.17) | |
| <6.8 | 3.6 | 0.67 (0.43, 1.04) | 0.71 (0.45, 1.12) | 4.0 | 0.59 (0.38, 0.90) | 0.63 (0.40, 0.98) | |
| Linear folate, /IQD | — | — | 1.13 (0.97, 1.33) | — | — | 1.23 (1.03, 1.47) | 0.494 |
| No. of B-vitamin deficiencies | >0.053 | ||||||
| 0 | 2.5 | 1.00 (reference) | 1.00 (reference) | 2.6 | 0.94 (0.69, 1.30) | 0.94 (0.68, 1.31) | |
| 1 | 3.5 | 0.63 (0.43, 0.92) | 0.61 (0.42, 0.90) | 3.5 | 0.62 (0.41, 0.93) | 0.61 (0.40, 0.95) | |
| ≥2 | 3.0 | 0.77 (0.45, 1.32) | 0.77 (0.44, 1.35) | 4.8 | 0.40 (0.25, 0.65) | 0.47 (0.28, 0.77) | |
Clinical pregnancy was defined as any pregnancy that lasted ≥6 wk (≥42 d) after the onset of the last menstrual period and was confirmed by using a human chorionic gonadotropin assay. The analysis was adjusted for maternal age, education, BMI, perceived life stress, tea drinking habit, occupational exposure to dust and noise, and husband smoking. The IQD was 159.3 pmol/L for vitamin B-12, 17.8 nmol/L for vitamin B-6, and 4.8 nmol/L for folate. CP, clinical pregnancy; DDT, 1,1,1-trichloro-2,2,bis(p-chlorophenyl)ethane; IQD, interquartile distance.
Time at which 50% of women had a clinical pregnancy was estimated on the basis of Weibull models. Delayed CP was represented by a longer median time to CP.
Estimated in the adjusted model with DDT (low or high) and vitamin (normal or deficiency) and the interaction of the two.
Estimated in the adjusted model with DDT (low or high) and vitamin (linear term) and the interaction of the two.