TABLE 3.
HRs summarizing the effect of preconception DDT and B-vitamin status on the incidence of the first observed conception 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 first conception, cycles2 | Crude | Adjusted | Median time to first conception, cycles2 | Crude | Adjusted | P-effect modification | |
| Vitamin B-12, pmol/L | 0.0023 | ||||||
| Normal | 2.4 | 1.00 (reference) | 1.00 (reference) | 2.1 | 1.17 (0.91, 1.52) | 1.18 (0.90, 1.54) | |
| <258 | 1.7 | 1.54 (1.02, 2.34) | 1.78 (1.15, 2.75) | 3.5 | 0.59 (0.36, 0.96) | 0.70 (0.42, 1.19) | |
| Linear vitamin B-12, /IQD | — | — | 0.77 (0.63, 0.93) | — | — | 1.32 (1.04, 1.67) | 0.00044 |
| Vitamin B-6, nmol/L | 0.313 | ||||||
| Normal | 2.2 | 1.00 (reference) | 1.00 (reference) | 2.1 | 1.08 (0.82, 1.41) | 1.08 (0.81, 1.44) | |
| <30 | 2.4 | 0.88 (0.59, 1.32) | 0.90 (0.59, 1.40) | 3.0 | 0.67 (0.47, 0.96) | 0.72 (0.50, 1.05) | |
| Linear vitamin B-6, /IQD | — | — | 0.99 (0.80, 1.23) | — | — | 1.47 (1.18, 1.83) | 0.014 |
| Folate, nmol/L | 0.443 | ||||||
| Normal | 2.1 | 1.00 (reference) | 1.00 (reference) | 2.3 | 0.90 (0.69, 1.17) | 0.93 (0.70, 1.22) | |
| <6.8 | 2.8 | 0.68 (0.44, 1.04) | 0.71 (0.46, 1.11) | 2.6 | 0.77 (0.52, 1.15) | 0.84 (0.56, 1.27) | |
| Linear folate, /IQD | — | — | 1.06 (0.90, 1.25) | — | — | 1.07 (0.90, 1.27) | 0.934 |
| No. of B-vitamin deficiencies | >0.053 | ||||||
| 0 | 2.2 | 1.00 (reference) | 1.00 (reference) | 2.1 | 1.06 (0.77, 1.45) | 1.09 (0.78, 1.51) | |
| 1 | 2.4 | 0.86 (0.60, 1.24) | 0.87 (0.60, 1.27) | 2.2 | 1.00 (0.68, 1.48) | 0.99 (0.66, 1.48) | |
| ≥2 | 2.2 | 1.02 (0.60, 1.72) | 1.19 (0.68, 2.07) | 3.2 | 0.60 (0.39, 0.94) | 0.72 (0.45, 1.16) | |
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. DDT, 1,1,1-trichloro-2,2,bis(p-chlorophenyl)ethane; IQD, interquartile distance.
Time at which 50% of women had a first conception during the follow-up period was estimated on the basis of Weibull models. Delayed first conception was represented by a longer median time to conception.
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.