TABLE 2.
Predictors of serum 25-hydroxyvitamin D in pregnant adolescents1
| Fixed effects | β ± SE | P value |
|---|---|---|
| Intercept | 23.1 ± 1.7 | <0.0001 |
| Gestational age,2 wk | −0.169±0.081 | 0.0423 |
| Season of delivery (spring-summer)3 | −0.396 ± 2.1 | 0.85 |
| Gestational age × season of delivery | 0.332 ± 0.11 | 0.0027 |
| Intake of intervention vitamin D,4 1000 IU/d | 5.69 ± 1.6 | 0.0007 |
| Gestational age × intake of intervention vitamin D5 | 0.222 ± 0.074 | 0.0043 |
| Maternal race (white/other)6 | 6.19 ± 2.0 | 0.0038 |
1Values are fixed-effects estimates from a random coefficients model of serum 25-hydroxyvitamin D concentration conducted using SAS PROC MIXED with an unstructured covariance matrix, n = 67 subjects, 194 observations. IU, International Units.
2Gestational age is centered at mean 28.7 wk.
3Effect of spring-summer (April–October) delivery compared with the reference category, fall-winter (November–March) delivery.
4Daily intake of vitamin D3 from the supplementation intervention is centered at the median 0.111 (i.e., 111 IU).
5The gestational age coefficient −0.1692 is the slope of gestational age when season is fall-winter and intake of intervention vitamin D is 0.111 (1000 IU/d). The slope of gestational age in fall-winter becomes nonnegative (i.e., is zero) when intake of intervention vitamin D is 0.873 (1000 IU/d) or 873 IU/d: 0 = 0.111 – (−0.1692 ÷ 0.2220) – X. Solve for X = 0.873 or 873 IU/d.
6Effect of white/other race compared with reference category, African American race.