Maternal calcium metabolic stress and fetal growth

Abstract

Background: Suboptimal maternal calcium intake and vitamin D status may or may not adversely influence fetal growth.

Objective: It was hypothesized that maternal calcium metabolic stress in early pregnancy, rather than suboptimal calcium intake or insufficient vitamin D, influences the risk of small-for-gestational-age (SGA) births and other aspects of fetal growth. Stress to calcium metabolism was defined as elevated intact parathyroid hormone (PTH) (>62 pg/mL) accompanied by a very low calcium intake [<60% of the Estimated Average Requirement (EAR)] or insufficient 25-hydroxyvitamin D [25(OH)D] (<20 ng/mL).

Design: This was a prospective cohort study of 1116 low-income and minority gravidae at entry to care of 13.8 ± 5.6 wk (mean ± SD).

Results: The PTH concentration depended on circulating 25(OH)D and total calcium intake. When 25(OH)D was insufficient, even a high calcium intake (which equaled or exceeded the Recommended Dietary Allowance) was unable to maintain PTH or to moderate the proportion of patients with an elevated PTH. When examined one at a time, very low calcium intake (<60% of EAR), very low 25(OH)D (<12 ng/mL), and elevated PTH (>62 pg/mL) each had a small but significant association with birth weight. Elevated PTH was also related to birth length and risk of SGA birth. Elevated PTH accompanied by insufficient 25(OH)D or very low calcium intake was associated with a 2- to 3-fold increased risk of SGA birth and a significantly lower birth weight, birth length, and head circumference, even after women who developed preeclampsia were excluded. Infants born to gravidae with insufficient 25(OH)D or very low calcium intake without elevated PTH or with elevated PTH alone were unaffected.

Conclusion: Maternal calcium metabolic stress, rather than low calcium intake or insufficient vitamin D, has an adverse influence on fetal growth. This trial was registered at clinicaltrials.gov as NIH 0320070046.

INTRODUCTION

Vitamin D and its metabolites, along with dietary calcium intake and parathyroid hormone (PTH)⁴, are involved in the regulation of serum calcium and calcium metabolism (1). A considerable body of data suggests that maternal vitamin D and calcium status are related to infant bone health during pregnancy, when the retention of an estimated 30 g Ca is needed for mineralization of the fetal skeleton (1). Calcium absorption approximately doubles in pregnant women, so that a suboptimal calcium intake usually does not adversely affect the maternal-fetal unit (1); however, the potential to do so may exist if calcium homeostasis is precarious before or during pregnancy (2, 3). In pregnant women, a calcium intake <600 mg/d results in a negative calcium balance; the risk of a negative balance increases at intakes below 800 to <1000 mg Ca/d (1, 3). Risk also increases with vitamin D deficiency or insufficiency (4).

Vitamin D helps maintain appropriate serum calcium concentrations by regulating the release of PTH to increase calcium absorption. There is an inverse relation between dietary calcium intake (suboptimal) and vitamin D (insufficient) to PTH (4). The consequences of too little calcium intake or insufficient vitamin D during pregnancy include an elevated PTH. An elevated PTH is generally regarded as a sign of stress to calcium metabolism (1, 5). The sequelae of calcium stress plausibly include decreased fetal growth (birth weight, birth length, and fetal femur length in pregnant adolescents) (6–8). Although the subject of little research, elevated PTH may adversely influence infant weight and length (9).

We hypothesized that stress to maternal calcium metabolism, rather than a low calcium intake or insufficient vitamin D status, influences fetal growth. Therefore, the effect of total calcium intake and 25-hydroxyvitamin D [25(OH)D] on the concentration of PTH was used as an indicator of calcium homeostasis (4). The relation of calcium intake, circulating 25(OH)D, and PTH to infant birth weight, risk of small-for-gestational-age (SGA) births, and other measures of fetal growth (birth length and head circumference) was also described. We used data from a large prospective study of vitamin D and preeclampsia that reported a positive association between PTH and maternal blood pressure and a >2-fold increase in risk of preeclampsia that was specific to secondary hyperparathyroidism (10). Because preeclampsia is associated with lower birth weight and an increased risk of fetal growth restriction (11) and with elevated PTH (10), data are reported before and after the exclusion of women who developed preeclampsia.

SUBJECTS AND METHODS

The Camden Study (September 1985 to present) is a prospective cohort study of pregnancy outcome and complications in young, generally healthy women residing in one of the poorest cities in the continental United States. We studied 1116 participants from the cohort with data on 25(OH)D, total calcium intake (from diet and supplements), and PTH who enrolled and delivered a live born infant between 2001 and 2007. The Institutional Review Board at the University of Medicine and Dentistry of New Jersey–School of Osteopathic Medicine approved the protocol. Informed written consent was obtained from each participant after the purpose of the study was explained. Subjects were recruited from among women presenting for prenatal care at the Osborn Family Health Center, Our Lady of Lourdes Medical Center (Camden, NJ) at entry to prenatal care (<20 wk of gestation). Women with serious nonobstetric problems (eg, lupus, type 1 or type 2 diabetes, seizure disorders, malignancies, acute or chronic liver or renal diseases, drug or alcohol abuse, and a psychiatric diagnosis) were not eligible.

Socioeconomic, demographic, lifestyle, and dietary data were obtained by in-person interviews at entry to prenatal care (<20 wk of gestation) and were updated at weeks 20 and 28. A 24-h recall of the previous day's diet was obtained on the same schedule. Food models were used to estimate portion size with dietary probes for forgotten foods and unspecified items [eg, if cream or milk (whole, 1%, 2%, skim) was used in coffee or tea]. The type, frequency, and duration of supplement use was obtained at each visit and from before pregnancy, including whether or not the supplement was prescribed. This method of assessing supplement use was previously validated by assaying the change in circulating micronutrients included in prenatal vitamin/mineral formulations (12). Data were processed by using databases from the Campbell Institute of Research and Technology (Campbell Soup Company) in Camden by using the US Department of Agriculture Nutrient Database for Standard Reference (http://www.nal.usda.gov/fnic/foodcomp) and the Continuing Survey of Food Intakes by Individuals (http://www.barc.usda.gov/bhnrc/foodsurvey/) and data from the scientific literature. Nutrient values from the 3 recalls, including total calcium intake (diet plus supplements), were averaged across the pregnancy, and the mean was used in this analysis. A total of 15% of total calcium intake came from supplements and 85% from diet. Intakes were validated by computing measures of reliability and by assay of biomarkers, including 25(OH)D (12–14).

Unlike food frequency, which is usually self-administered for cost-effectiveness, the 24-h recall has a low respondent burden. It makes no demands on the ability to read, write, or use a computer and consequently does not exclude women whose education or abilities are limited. Whereas the 24-h recall estimates diet in the short term, studies including those of low-income pregnant women show correlations of approximately r = 0.5 for calcium and other nutrients when food-frequency questionnaires and three 24-h recalls are compared (15–17). Self-reported diets underestimate caloric intake when judged against energy expenditure by the doubly labeled water method (17). Thus, it is possible that calcium intake was also underestimated.

BMI was computed [weight (kg)/height (m)²] from self reported pregravid weight and height measured with a stadiometer at entry to care. Pregravid weight is stated accurately by most women (18). Maternal ethnicity [African American, Hispanic (including black and white women) and non-Hispanic white] was self-reported.

Infant birth weight, length, and head circumference, gestation duration, sex, and other relevant variables were abstracted from the prenatal record, the delivery record, delivery logbooks, and the infant's chart. Gestation duration was based on the mother's last menstrual period, which was confirmed or modified by ultrasonography. Fetal growth restriction, indexed by birth of an infant that is SGA, was defined as below the 10th percentile of standards of birth weight for gestational age that control for maternal ethnicity, parity, and fetal sex (19). Preeclampsia was defined by gestational hypertension after 20 wk of gestation (systolic blood pressure >140 mm Hg systolic or diastolic blood pressure >90 mm Hg in a normotensive woman) accompanied by new-onset proteinuria (≥1+ by dipstick) (10, 11).

Maternal serum obtained at entry was stored at −70° and used to assay intact PTH and 25(OH)D. Serum intact PTH was measured by immunoradiometric assay (DSL Diagnostic Systems Laboratories Inc). The overall intra- and interassay CVs were <5%. Circulating 25(OH)D was measured in serum as 25-hydroxyvitamin D₃ and 25-hydroxyvitamin D₂ by HPLC in serum by using a kit marketed by Chromsystems. The within-assay and between-assay CVs were <8%. Available data show that 25(OH)D and PTH are stable and reproducible for several years when stored at −70°C (20, 21).

Our prior research suggests that PTH declined then reached a plateau when concentrations of 25(OH)D exceed 19 ng/mL (10, 13)—a finding consistent with recent Institute of Medicine guidelines for vitamin D insufficiency (1). We therefore examined concentrations of 25(OH)D at entry that were <20 ng/mL (<12, 12–15.9, and 16–19.9), comparing them with concentrations ≥20 ng/mL. Elevated PTH at entry was defined by a concentration that exceeded the upper reference limit for the assay (>62 pg/mL, or 6.82 pmol/L) (5). Values for total calcium intake (diet plus supplements) were averaged from the 3 self reports taken across the pregnancy and the mean used in this analysis. Total calcium intake was categorized as far below(<60%) or below (60% to <100%) the Estimated Average Requirement (EAR) for pregnancy (<1000 mg/d at age <19 y, <800 mg/d at age ≥19 y), meeting the EAR but below the Recommended Dietary Allowance (RDA; 1000 to <1300 mg/d at age <19 y, 800 to <1000 mg/d at age ≥19 y), or meeting or exceeding the RDA (≥1300 mg/d at age <19 y, ≥1000 mg/d at age ≥19 y) for pregnancy (1). We used calcium intake at <60% of EAR because intake is likely underestimated in self reports of diet (1, 17). At <60% of EAR for calcium, we were able to study a large enough number of women to detect small to moderate effects with a statistical power close to 80%.

ANOVA was used to examine the influence of low calcium intake (<EAR) and insufficient 25(OH)D (<20 ng/mL) at entry on the concentration of entry PTH (4). PTH concentrations of women with calcium intakes below the EAR were compared with those with higher intakes when 25(OH)D was sufficient (≥20 ng/mL) or insufficient (<20 ng/mL). We began with the most extreme groups (<60% of EAR compared with ≥RDA); planned comparisons ceased at the first nonsignificant difference. The difference in PTH between vitamin D–sufficient and –insufficient gravidae was also examined according to calcium intake in post hoc testing by using Bonferroni's correction.

Multiple linear regression was also used to examine the influence of dietary calcium intake, 25(OH)D, and elevated PTH separately on infant birth weight for gestation, length, and head circumference. Multiple logistic regression was used to fit separate models for SGA with low calcium intake, insufficient 25(OH)D, and elevated PTH at entry. In linear and logistic regression analyses, the group with a calcium intake regarded as optimal (≥RDA), with concentrations of 25(OH)D regarded as sufficient (≥20 ng/mL), or with a concentrations of PTH that were within the normal limits for the assay (<62 pg/mL) was used as the reference category. We again began with a comparison of the most extreme groups; these planned comparisons ceased at the first difference that was not significant.

Effects consistent with maternal calcium metabolic stress to fetal growth were also examined. Because outcomes combining elevated PTH with low calcium intake were similar to outcomes combining elevated PTH with insufficient 25(OH)D, we put them together and compared 1) gravidae with elevated PTH having either (or both) insufficient 25(OH)D (<20 ng/mL) or very low dietary calcium intake (<EAR or <60% of EAR), 2) gravidae without elevated PTH having either (or both) insufficient 25(OH)D or low calcium intake, and 3) gravidae with elevated PTH having either (or both) a higher calcium intake (≥EAR or ≥60% of EAR) or sufficient 25(OH)D (≥20 ng/mL) with controls without these risk factors (calcium intake ≥EAR or ≥60% of EAR, 25(OH)D ≥20 ng/mL, and PTH ≤62).

Potential confounders were defined as those that altered the adjusted OR (AOR) or regression coefficient by ≥10%, and assessed by comparing crude and adjusted coefficients. We excluded season as a confounder because seasonal fluctuations in PTH and 25(OH)D, and not other factors that varied by season, were expected to influence fetal growth. AORs and their 95% CIs were computed from the logistic regression coefficients and their corresponding covariance matrices. Data were analyzed with SAS version 9.0 (SAS Institute).

RESULTS

Background characteristics, including dietary calcium intake (adjusted for energy) and concentrations of 25(OH)D and PTH, are described in Table 1 for young (22.8 ± 5.5 y) parous (61.9%) gravidae at entry to care (13.8 ± 5.6 wk of gestation). Infant birth weight was 3179 ± 607 g, and the gestation duration was 38.5 ± 2.5 wk.

TABLE 1.

Background characteristics of participants¹

Characteristic	Value
Age (y)	22.8 ± 5.52
Age group [n (%)]
<19 y	260 (23.3)
19–24.9 y	506 (45.3)
25–29.9 y	203 (18.2)
≥30 y	147 (13.2)
Parity [n (%)]
Parous	690 (61.8)
Nulliparous	426 (38.2)
Ethnicity [n (%)]
Hispanic	574 (51.4)
Non-Hispanic black	384 (34.4)
Non-Hispanic white	158 (14.2)
Smoking [n (%)]
No	890 (79.8)
Yes	226 (20.2)
BMI (kg/m2)	26.0 ± 6.4
BMI group [n (%)]
<25 kg/m2	607 (54.4)
25–29.9 kg/m2	226 (20.2)
≥30 kg/m2	283 (25.4)
Entry 25(OH)D (ng/mL)	26.9 ± 12.9
25(OH)D group [n (%)]
<12 ng/mL	119 (10.7)
12–15.9 ng/mL	112 (10.0)
16–19.9 ng/mL	148 (13.3)
≥20 ng/mL	737 (66.0)
Entry PTH (pg/mL)	36.5 ± 21.6
PTH group [n (%)]
>62 pg/mL	125 (11.2)
≤62 pg/mL	991 (88.8)
Total calcium intake (mg/d)3	1085 ± 447
Calcium group [n (%)]4
<60% of EAR	152 (13.6)
60% to <100% of EAR	197 (17.6)
100% of EAR, <100% of RDA	254 (22.8)
≥100% of RDA	513 (46.0)

¹EAR, Estimated Average Requirement; PTH, parathyroid hormone; RDA, Recommended Dietary Allowance; 25(OH)D, 25-hydroxyvitamin D.

²Mean ± SD (all such values).

³Adjusted for energy.

⁴Age-related guidelines.

When vitamin D was insufficient [25(OH)D <20 ng/mL], the risk of an elevated PTH concentration was uniformly high and did not differ by total calcium intake regardless of whether the intake was below the EAR or exceeded the RDA (Table 2). When vitamin D was sufficient (≥20 ng/mL), the risk of elevated PTH varied with calcium intake. Similarly, the lowest PTH concentration was found among women with the highest intake (≥RDA) and the highest among those with the lowest intake (<60% of EAR). When vitamin D was sufficient, PTH was significantly different only between the extremes (<60% of EAR compared with ≥RDA for calcium). When examined by calcium intake, the concentration of PTH was not dissimilar (P = 0.18) between vitamin D–sufficient and –insufficient women with intakes at <60% of the EAR of calcium. PTH, however, was significantly lower in vitamin D–sufficient women whose calcium intake exceeded 60% of the EAR, equaled the EAR, and equaled or exceeded the RDA than in vitamin D–insufficient women with the same intake of calcium. Moreover, a 3-fold increased risk of elevated PTH (>62 pg/mL) was observed in vitamin D–insufficient gravidae, regardless of their calcium intake, and a 2-fold increase in risk was observed in vitamin D–sufficient women, but only when the calcium intake was very low (<60% of EAR) (Table 2).

TABLE 2.

25(OH)D and PTH at entry and total calcium intake¹

		Entry PTH			Elevated PTH
Total calcium intake (energy adjusted)	No. of subjects	Unadjusted2	Adjusted23	Elevated (>62 pg/mL)	OR	95% CI	AOR4	95% CI
		pg/nL	pg/nL	%
Entry 25(OH)D: <20 ng/nL
Calcium <60% of EAR	72	45.2 ± 2.5	45.5 ± 2.3	16.7	3.255	1.4, 7.3	3.495	1.5, 8.1
Calcium 60% to <100% of EAR	76	43.7 ± 2.46	42.4 ± 2.46	17.1	3.255	1.5, 7.0	3.055	1.4, 6.7
Calcium 100% of EAR and <100% of RDA	84	42.4 ± 2.36	41.0 ± 2.26	20.2	3.817	1.9, 7.6	3.547	1.8, 7.1
Calcium ≥100% of RDA	147	43.6 ± 1.76	42.6 ± 1.86	19.0	3.357	1.9, 6.0	3.37	1.8, 6.0
Entry 25(OH)D: ≥20 ng/nL
Calcium <60% of EAR	80	38.3 ± 2.38	38.9 ± 2.18	12.5	2.31	0.99, 5.3	2.49	1.0, 5.6
Calcium 60% to <100% of EAR	121	33.7 ± 1.9	34.7 ± 1.9	8.3	1.41	0.63, 3.1	1.53	0.68, 3.43
Calcium 100% of EAR and <100% of RDA	170	33.1 ± 1.6	33.7 ± 1.6	6.5	1.05	0.49, 2.2	1.10	0.51, 2.37
Calcium ≥100% of RDA	366	31.1 ± 1.1	31.2 ± 1.1	6.5	Reference		Reference

¹AOR, adjusted OR; EAR, Estimated Average Requirement; PTH, parathyroid hormone; RDA, Recommended Dietary Allowance; 25(OH)D, 25-hydroxyvitamin D.

²Values are means ± SEMs.

³Adjusted for age, parity, ethnicity, BMI, smoking, and energy (ANOVA); P-interaction ≤ 0.001.

⁴Adjusted for age, parity, ethnicity, BMI, smoking, and energy (logistic regression); P-trend < 0.001.

⁵P < 0.005 compared with calcium ≥RDA, where 25(OH)D ≥20 ng/mL (logistic regression).

⁶P ≤ 0.05 compared with PTH in the same calcium category, where 25(OH)D ≥20 ng/mL (t test with Bonferroni correction).

⁷P < 0.001 compared with calcium ≥RDA, where 25(OH)D ≥20 ng/mL (logistic regression).

⁸P ≤ 0.006 compared with calcium ≥RDA, where 25(OH)D ≥20 ng/mL (t test).

⁹P < 0.05 compared with calcium ≥RDA, where 25(OH)D ≥20 ng/mL (logistic regression).

When examined in separate models, very low calcium intake (<60% of EAR), very low 25(OH)D (<12 ng/mL), and elevated PTH (>62 pg/mL) were all related to reduced infant birth weight for gestation compared with birth weights of infants born to gravidae with optimal calcium intakes (≥RDA), sufficient 25(OH)D (≥20 ng/mL), or PTH within assay limits (<62 pg/mL). Differences in birth weight were ∼70–95 g less for calcium and 25(OH)D and ∼76–105 g less for elevated PTH when confounding variables were controlled for (Table 3). When women with preeclampsia were excluded, very-low 25(OH)D (<12 ng/mL) and birth weight were no longer related. Neither calcium intake nor 25(OH)D was associated with changes in infant length or head circumference (data not shown). There was, however, a tendency for infants born to gravidae with elevated PTH to be 0.4–0.5 cm shorter than infants of other mothers [−0.50 ± 0.23 cm; P = 0.028 excluding gravidae with preeclampsia, −0.41 ± 0.23 cm, P = 0.060 for all gravidae (ANOVA)] after control for confounding.

TABLE 3.

Infant birth weight for gestation: very low calcium intake, insufficient 25(OH)D, and elevated PTH¹

	All women				Preeclampsia excluded
	Model 12		Model 23		Model 12		Model 23
	b ± SE	P	b ± SE	P	b ± SE	P	b ± SE	P
	g		g		g		g
Very low calcium intake (energy adjusted)
<60% of EAR	−72.6 ± 46.7	0.09	−96.0 ± 43.7	0.03	−73.0 ± 48.3	0.13	−94.9 ± 44.3	0.03
60% to <100% of EAR	−19.9 ± 42.8	NS	−40.8 ± 37.6	NS	−1.9 ± 44.0	NS	−33.6 ± 37.9	NS
100% of EAR and <100% of RDA	16.9 ± 36.6	NS	−34.5 ± 32.9	NS	35.7 ± 37.6	NS	39.1 ± 33.5	NS
≥100% of RDA	Reference		Reference		Reference		Reference
Insufficient 25(OH)D
<12 ng/mL	−77.8 ± 41.1	0.058	−84.1 ± 41.1	0.04	−62.3 ± 42.8	0.14	−62.5 ± 42. 6	0.14
12–15.9 ng/mL	−18.2 ± 41.5	NS	−49.4 ± 41.6	NS	−35.6 ± 43.3	NS	−70.5 ± 43.4	NS
16–19.9 ng/mL	16.1 ± 37.0	NS	11.4 ± 36.5	NS	22.9 ± 37.6	NS	17.3 ± 37.0	NS
≥20 ng/mL	Reference		Reference		Reference		Reference
PTH
≤62 pg/nL	Reference		Reference		Reference		Reference
>62 pg/nL	−34.1 ± 39.3	0.38	−76.0 ± 38.6	0.049	−64.4 ± 41.0	0.11	−105.0 ± 40.1	0.009

¹EAR, Estimated Average Requirement; PTH, parathyroid hormone; RDA, Recommended Dietary Allowance; 25(OH)D, 25-hydroxyvitamin D.

²Adjusted for gestation duration; n = 1116 (linear regression).

³Adjusted for gestation duration, age, parity, ethnicity, smoking, and BMI; n = 1116. Preeclampsia excluded: n = 1044 (calcium model). n = 1045 for 25(OH)D and PTH models (linear regression).

Neither very low calcium intake (<60% of EAR or <EAR) nor insufficient 25(OH)D (<20 ng/mL) was associated with risk of SGA (data not shown). On the other hand, when PTH was elevated, risk of SGA increased 2-fold for all women [11.3% compared with 6.7%; AOR = 2.01; 95% CI: 1.08, 3.75 (logistic regression)] as well as for those without preeclampsia [12.6% compared with 6.1%; AOR = 2.59; 95% CI: 1.37, 4.89 (logistic regression)] after control for confounding. Examination of the combined effects showed that gravidae with elevated PTH with either (or both) 25(OH)D <20 ng/mL or very low calcium intake (<60% of EAR) (15% had both) had a >2-fold to a >3-fold increase in risk of SGA, particularly if they did not develop preeclampsia (Table 4). Gravidae with either (or both) insufficient 25(OH)D or very low calcium (15.8% had both) without elevated PTH and gravidae with elevated PTH, sufficient 25(OH)D (≥20 ng/mL), and a higher calcium intake (≥60% of EAR) (100% had both) did not sustain an increased risk of SGA (Table 4). Findings with calcium <EAR were comparable.

TABLE 4.

SGA: influence of elevated PTH and insufficient 25(OH)D or very low calcium intake¹

	All women								Preeclampsia excluded
Groups	No. of subjects	SGA	OR	95% CI	P	AOR2	95% CI	P	No. of subjects	SGA	OR	95% CI	P	AOR2	95% CI	P
		%								%
PTH >62, 25(OH)D <20, or calcium <60% of EAR3	79	15.14	2.23	1.12, 4.43	0.02	2.51	1.24, 5.09	0.01	69	17.39	2.88	1.43, 5.82	0.003	3.26	1.58, 6.73	0.001
PTH ≤62, 25(OH)D <20, or calcium <60% of EAR4	384	4.95	0.66	0.38, 1.14	NS	0.68	0.39, 1.19	NS	360	4.44	0.65	0.35, 1.17	NS	0.67	0.36, 1.22	NS
PTH >62, 25(OH)D ≥20, or calcium ≥60% of EAR5	45	4.44	0.55	0.13, 2.35	NS	0.64	0.15, 2.74	NS	42	4.76	0.86	0.20, 3.71	NS	0.80	0.18, 3.45	NS
PTH ≤62, 25(OH)D ≥20, or calcium ≥60% of EAR5	606	7.76	Ref			Ref			574	7.14	Ref			Ref

¹AOR, adjusted OR; EAR, Estimated Average Requirement; PTH, parathyroid hormone; Ref, reference; SGA, small for gestational age; 25(OH)D, 25-hydroxyvitamin D.

²Adjusted for age, BMI, smoking, and energy (logistic regression). SGA standard controls gestational age, maternal parity, ethnicity, and fetal sex; P-interaction = 0.010.

³15% had both 25(OH)D <20 ng/mL and calcium <60% of EAR.

⁴15.6% had both 25(OH)D <20 ng/mL and calcium <60% of EAR.

⁵100% had both 25(OH)D ≥20 ng/mL and calcium ≥60% of EAR.

Similar results were found for elevated PTH, very low calcium intake, and insufficient 25(OH)D in relation to infant birth weight, birth length, and head circumference (Table 5). Birth weight was reduced by ∼102–128 g, birth length by 0.5–0.6 cm, and head circumference by 0.40 cm for infants born to women with elevated PTH and either (or both) insufficient 25(OH)D or very low calcium intake. Insufficient 25(OH)D or very low calcium intake without elevated PTH and elevated PTH with sufficient 25(OH)D (≥20 ng/mL) or higher calcium intake (≥60% of EAR) did not influence these measures of fetal growth (Table 5). Findings obtained by using calcium intake <EAR were similar.

TABLE 5.

Infant birth weight, birth length, and head circumference for gestation: influence of elevated PTH with insufficient 25(OH)D or very low calcium intake¹

	All women				Preeclampsia excluded
	Model 12		Model 23		Model 12		Model 23
	b ± SE	P	b ± SE	P	b ± SE	P	b ± SE	P
Birth weight (g)
PTH >62, 25(OH)D <20, or calcium <60% of EAR4	−52.1 ± 47.1	0.26	−102.2 ± 47.0	0.024	−93.5 ± 49.7	0.06	−128.5 ± 49.9	0.009
PTH ≤62, 25(OH)D <20, or calcium <60% of EAR5	−7.0 ± 26.3	NS	−22.8 ± 27.3	NS	−1.6 ± 26.8	NS	−6.2 ± 26.6	NS
PTH >62, 25(OH)D ≥20, or calcium ≥60% of EAR6	−7.0 ± 74.5	NS	−43.4 ± 72.8	NS	5.4 ± 75.0	NS	−49.4 ± 73.2	NS
PTH ≤62, 25(OH)D ≥20, or calcium ≥60% of EAR6	Reference		Reference		Reference		Reference
Birth length (cm)
PTH >62, 25(OH)D <20, or calcium <60% of EAR4	−0.59 ± 0.35	0.09	−0.53 ± 0.27	0.042	−0.54 ± 0.37	0.014	−0.63 ± 0.28	0.026
PTH ≤62, 25(OH)D <20, or calcium <60% of EAR5	0.07 ± 0.20	NS	0.12 ± 0.15	NS	0.16 ± 0.21	NS	0.19 ± 0.16	NS
PTH >62, 25(OH)D ≥20, or calcium ≥60% of EAR6	0.12 ± 0.54	NS	0.13 ± 0.40	NS	0.13 ± 0.54	NS	0.12 ± 0.41	NS
PTH ≤62, 25(OH)D ≥20, or calcium ≥60% of EAR6	Reference		Reference		Reference		Reference
Head circumference (cm)
PTH >62, 25(OH)D <20, or calcium <60% of EAR4	−0.36 ± 0.24	0.13	−0.39 ± 0.20	0.047	−0.29 ± 0.25	0.24	−0.42 ± 0.21	0.045
PTH ≤62, 25(OH)D <20, or calcium <60% of EAR5	−0.07 ± 0.14	NS	−0.07 ± 0.11	NS	−0.03 ± 0.14	NS	−0.05 ± 0.12	NS
PTH >62, 25(OH)D ≥20, or calcium ≥60% of EAR6	−0.07 ± 0.37	NS	−0.01 ± 0.28	NS	0.08 ± 0.37	NS	−0.01 ± 0.30	NS
PTH ≤62, 25(OH)D ≥20, or calcium ≥60% of EAR6	Reference		Reference		Reference		Reference

¹EAR, Estimated Average Requirement; PTH, parathyroid hormone; 25(OH)D, 25-hydroxyvitamin D.

²Adjusted for gestation duration (linear regression).

³Adjusted for gestation duration, age, parity, ethnicity, smoking, and BMI (linear regression). Birth weight: n = 1116 all women, n = 1049 excluding preeclampsia. Birth length: n = 1034, 967 excluding preeclampsia. Head circumference: n = 1032, 965 excluding preeclampsia. P-interaction = 0.05 (birth weight and length), P = 0.16 (head circumference).

⁴15% had both 25(OH)D <20 ng/mL and calcium <60% of EAR.

⁵15.6% had both 25(OH)D <20 ng/mL and calcium <60% of EAR.

⁶100% had both 25(OH)D ≥20 ng/mL and calcium ≥60% of EAR.

DISCUSSION

The effects of total calcium intake and 25(OH)D on the concentration of PTH were used to define stress to maternal calcium metabolism. We found that it was maternal calcium metabolic stress, rather than low calcium intake or insufficient vitamin D status, that was associated with a greater risk of SGA and it had an adverse influence on other measures of fetal growth (birth weight, length, and head circumference).

When vitamin D was insufficient, even a high calcium intake equivalent to the RDA for pregnancy was unable to maintain the concentration of PTH or to moderate the proportion with elevated PTH. When vitamin D was sufficient, the concentration of PTH and the proportion with elevated PTH (>62 pg/mL) varied with maternal calcium intake. Nevertheless, when vitamin D was sufficient but calcium intake was very low, PTH increased beyond the concentration associated with a high calcium intake (≥RDA) and was similar to concentrations found in vitamin D–insufficient gravidae. In addition, the risk of elevated PTH was increased 3-fold when vitamin D was insufficient, regardless of calcium intake, and increased 2-fold among vitamin D–sufficient women with a very low calcium intake (<60% of EAR). These results by and large confirm results from a cross-sectional study of Icelandic adults (4) and extend the most important conclusions to pregnant and minority women from Camden: that both vitamin D and calcium intake are important to calcium metabolism.

Our results also suggest that maternal calcium metabolic stress rather than insufficient 25(OH)D or a low calcium intake was associated with reduced fetal growth. When examined one at a time, very low calcium intake (<60% of EAR), very low 25(OH)D (<12 ng/mL), and elevated PTH (>62 pg/mL) each had a small but significant association with infant birth weight. Elevated PTH was also associated with reduced infant birth length and an with increased risk of SGA. When the combined effects of PTH, calcium intake, and 25(OH)D were examined in detail, there was an increased risk of SGA and a significantly lower infant birth weight, birth length, and head circumference in infants born to gravidae with elevated PTH and either (or both) 25(OH)D (<20 ng/mL) or low calcium intake (<60% of EAR). Infants born to gravidae with insufficient 25(OH)D or very-low calcium intake without elevated PTH or with elevated PTH alone were unaffected.

Elevated PTH with insufficient vitamin D or low calcium intake—known as secondary hyperparathyroidism—is regarded as a sign of stress to calcium metabolism (1, 5, 22). PTH synthesis and secretion is normally regulated by serum calcium concentrations through negative feedback. PTH increases the reabsorption of calcium and decreases calcium clearance in the distal renal tubules. PTH can also increase 1,25-dihydroxyvitamin D [1,25(OH)₂D] synthesis. This is the active form of vitamin D, which enhances the absorption of calcium and phosphorus in the intestine. Both hormones then act to raise serum calcium which in turn decreases PTH secretion (1, 5, 22).

Whereas the consequences of too little dietary calcium intake during pregnancy plausibly include decreased fetal growth (birth weight and length), few studies (7, 23, 24) have showed such effects. Likewise, whereas findings from observational studies of dietary vitamin D and circulating 25(OH)D or clinical trials in which pregnant women were supplemented with vitamin D usually show increased circulating maternal 25(OH)D (6, 25), only a small number have reported alterations in birth weight (24, 26–28), an increased risk of SGA (27, 29), or other differences in other measures of fetal growth (8, 30).

Calcium and vitamin D, however, have most often been examined independently; few studies have examined simultaneous effects on fetal growth or measured PTH. For example, Mannion et al (24) described the relation between restricted milk drinking (calcium + vitamin D) and lower infant birth weight (∼80 g) and showed that both milk consumption and vitamin D intake were significant predictors of birth weight. In a study of pregnant adolescents, maternal calcium intake was positively associated with birth length, but only when gravidae were also 25(OH)D insufficient (<20 ng/mL) (23). A study of vitamin D–deficient Pakistani gravidae (9), one-third of whom also had elevated PTH (>50 pg/mL), reported infant birth length and birth weight to be correlated inversely with maternal PTH, directly with serum ionized calcium but unrelated to 25(OH)D. Thus, whereas maternal vitamin D deficiency, by itself, did not impair fetal growth, the investigators suggested that it did so indirectly by disturbing maternal calcium homeostasis (9). Our results are consistent with this conclusion.

Disturbed calcium homeostasis in the mother is associated with calcium metabolic stress to the fetus (31). Case reports suggest that maternal hypercalcemia is able to suppress growth of the fetal parathyroid gland and to trigger fetal hypocalcemia (32). Maternal hypocalcemia has the opposite effect—it is thought to increase fetal PTH and PTH-related peptide (31) and bring about poorly mineralized bone (32) or a smaller fetal cortical bone envelope (31). Thus, calcium metabolic stress plausibly influences infant size at birth, in part by altering the fetal skeleton. SGA infants have a smaller skeleton and a lower bone mineral content (33). In appropriate-for-gestation infants, weight measured at or close to delivery correlates inversely with total-bone mineral content (34, 35) and with bone mineral content or density during early and later childhood (36). A randomized controlled trial (2 g elemental calcium/d compared with placebo) showed a trend toward increased infant birth weight (+121 g), length (+0.5 cm), and head circumference (+0.3 cm) among the calcium-supplemented gravidae, but differences were not significant, in part because of a small sample size (n = 128 per group) (37).

PTH, another component of maternal calcium metabolism, is also linked to reduced fetal growth. Elevated PTH is related to higher maternal blood pressure and risk of preeclampsia (10). Many children born after a hypertensive pregnancy have a lower birth weight for gestation (38); likewise, preeclampsia is associated with risk of intrauterine growth restriction and lower birth weight (11). Apart from hypertension, higher maternal diastolic blood pressure is associated with shorter first-trimester crown-rump length, which in turn is correlated with smaller second and third trimester head circumference, shorter femur length. and lower estimated fetal weight (39). The relation between maternal diastolic blood pressure and birth weight is inverted and U-shaped, with optimal birth weights occurring between 70 and 80 mm Hg. Maternal diastolic blood pressures above or below the optimal range are associated with lower birth weight and higher perinatal mortality (40).

This research had many limitations, including the use of routine clinical anthropometric measures for birth weight, length, and head circumference, which has more inherent variability than measures collected for research purposes. In a recent study, higher 1,25(OH)₂D in early as opposed to later pregnancy was associated with greater total calcium availability (the total exchangeable calcium pool); maternal calcium intake correlated positively with the rate of bone calcium deposition (41). We did not directly measure maternal calcium balance, ionized calcium, or the active form of vitamin D—1,25(OH)₂D. We used a combination of elevated PTH and low calcium intake or 25(OH)D to define stress to calcium metabolism and likely misclassified women (5). An important corollary of this research is to examine whether and to the extent to which maternal-fetal skeletal integrity is compromised by maternal calcium metabolic stress.