Maternal vitamin D status and calcium intake interact to affect fetal skeletal growth in utero in pregnant adolescents

Abstract

Background: Maternal calcium intake and vitamin D status may affect fetal bone development.

Objective: This study was designed to examine relations between maternal calcium intake, 25-hydroxyvitamin D [25(OH)D] status, and fetal bone growth across pregnancy.

Design: This was a prospective longitudinal design. Maternal 25(OH)D, parathyroid hormone, and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] were determined at midgestation (∼26 wk) and at delivery in 171 adolescents (≤ 18 y). Dietary recalls and fetal sonograms were performed up to 3 times across gestation, and fetal femur and humerus z scores were generated.

Results: Fetal femur and humerus z scores and neonatal birth length were significantly greater (P < 0.03) in adolescents consuming ≥ 1050 mg than in those consuming <1050 mg Ca/d. Maternal 25(OH)D > 50 nmol/L was significantly positively associated with fetal femur and humerus z scores (P < 0.01). When maternal smoking, height, race, weight gain, and gestational age were controlled for, these relations remained significant. Interactions between calcium intake and 25(OH)D were evident. Calcium intake was associated with fetal femur z scores and birth length only when maternal 25(OH)D was ≤ 50 nmol/L (P < 0.05). Similarly, maternal 25(OH)D was associated with fetal femur and humerus z scores only when maternal calcium intake was <1050 mg/d (P < 0.03).

Conclusions: Optimal calcium intake and adequate maternal vitamin D status are both needed to maximize fetal bone growth. Interactions between these nutrients were evident when either calcium or vitamin D status was limited. Improving maternal calcium intake and/or vitamin D status during pregnancy may have a positive effect on fetal skeletal development in pregnant adolescents.

FIGURE 1.

Fetal femur and humerus lengths were evaluated by sonogram at 33.8 ± 4.0 wk gestation in 169 pregnant adolescents, and z scores were calculated for each measure by using published equations (13, 21). Calcium intake was categorized into tertiles for fetal femur and humerus z scores, respectively: 1 <653 mg/d (n = 52, 50), 2 = 653–1066 mg/d (n = 52, 52), and 3 >1066 mg/d (n = 57, 57). A: Adolescents with a calcium intake within tertile 3 had better fetal femur length z scores (ANOVA; ^*P < 0.039) and tended to have better fetal humerus length z scores (ANOVA, ^#P < 0.070) than did those in calcium intake tertiles 1 and 2. B: Adolescents consuming ≥1050 mg Ca/d (n = 61) had significantly higher fetal femur (P = 0.020) and humerus (P = 0.032) z scores than did those consuming <1050 mg Ca/d (n = 100; 98 for fetal femur and humerus z scores, respectively).

FIGURE 2.

Third-trimester (33.8 ± 4.0 wk gestation) fetal femur and humerus lengths were measured and converted into z scores. Maternal dietary calcium intake and 25(OH)D status interacted to affect bone length outcomes. In adolescents with 25(OH)D ≤ 50 nmol/L, those who consumed ≥1050 mg Ca/d had significantly higher fetal femur z scores (*P = 0.019) and tended to have higher fetal humerus z scores (P = 0.116). Among adolescents who were 25(OH)D sufficient, no significant difference in fetal z scores was found between adolescents who consumed >1050 or <1050 mg Ca/d. 25(OH)D, 25-hydroxyvitamin D.

FIGURE 3.

An interaction between maternal 25(OH)D and maternal calcium intake was observed. Adolescents consuming <1050 mg Ca/d and those with 25(OH)D >50 nmol/L had significantly higher fetal femur (**P = 0.001) and humerus (*P = 0.030) z scores. Among adolescents who consumed ≥1050 mg Ca/d, no significant differences in fetal long bone z scores were found between adolescents who were 25(OH)D sufficient and those who were insufficient. 25(OH)D, 25-hydroxyvitamin D.

FIGURE 4.

An analogous interaction between maternal 25(OH)D and maternal calcium intake was also observed when calcium intake was partitioned by the Estimated Average Requirement for this age group (1100 mg/d). In adolescents consuming <1100 mg Ca/d, those with 25(OH)D >50 nmol/L had significantly higher fetal femur (*P = 0.019) and humerus (*P = 0.025) z scores, Among adolescents who consumed ≥1100 mg Ca/d, no significant differences in fetal long-bone z scores were found between adolescents who were 25(OH)D sufficient and those who were insufficient. 25(OH)D, 25-hydroxyvitamin D.

FIGURE 5.

Adolescents consuming calcium intakes <1100 mg/d who also had a 25(OH)D concentration ≤50 nmol/L (Ca/D insufficient) were compared with adolescents with a calcium intake ≥1100 mg/d and/or a 25(OH)D concentration >50 nmol/L (Ca/D sufficient). Adolescents within the Ca/D sufficient category (n = 94 and 93, respectively) had higher fetal femur and humerus z scores (*P = 0.002 and P = 0.003, respectively) than did those in the Ca/D insufficient category (n = 66 and 65, respectively). 25(OH)D, 25-hydroxyvitamin D.