1. Objectives

To evaluate whether oral vitamin D supplementation, alone or with calcium or other micronutrients, during pregnancy can safely improve maternal and infant outcomes

2. How studies were identified

The Cochrane Pregnancy and Childbirth Group Specialized Register was searched in February 2015, which includes records identified from the following databases:

• CENTRAL (The Cochrane Library)
• MEDLINE
• EMBASE
• CINAHL

The WHO International Clinical Trials Registry Platform and the Networked Digital Library of Theses and Dissertations were also searched, and handsearches of relevant journals and conference proceedings were performed. The authors also directly contacted relevant agencies and organizations in the field to identify ongoing and unpublished studies

3. Criteria for including studies in the review

3.1 Study type

Randomized controlled trials or quasi-randomized controlled trials

3.2 Study participants

Pregnant women of any gestational or chronological age, with any number of previous pregnancies or births

(Pregnant women with pre-existing conditions, such as gestational diabetes, were excluded)

3.3 Interventions

Oral vitamin D supplementation during pregnancy, at any dose, duration, or time of initiation, i) compared to no treatment or placebo, ii) in combination with calcium supplementation and compared to no treatment or placebo, iii) in combination with calcium supplementation and compared to calcium alone, and iv) in combination with calcium and other micronutrients and compared to supplementation with calcium and other micronutrients excluding vitamin D

3.4 Primary outcomes

Maternal

• Pre-eclampsia
• Gestational diabetes
• Vitamin D concentration at term (serum or plasma 25-hydroxyvitamin D in nmol/L)
• Adverse effects

Infant

• Preterm birth (<37 weeks’ gestation)
• Low birth weight (<2500 g)

Secondary outcomes included maternal impaired glucose tolerance, Caesarean section, gestational hypertension, maternal death (while pregnant or within 42 days of termination of pregnancy), birth length, head circumference at birth, birth weight, admission to intensive care unit during the neonatal period (within 28 days of delivery), stillbirth, neonatal death, Apgar score less than seven at five minutes, neonatal infection, and very preterm birth (<34 weeks’ gestation)

4. Main results

4.1 Included studies

Fifteen randomized controlled trials, enrolling 2833 women, were included in this review:

• In all trials, women were recruited and initiated supplementation at ≥20 weeks’ gestation
• One of the British studies was in first generation immigrants from Bangladesh, East Africa, India, Mauritius, Pakistan and Sri Lanka, while the other British study described participants’ ethnicities as Asian, Black, Caucasian, Indian, or Middle Eastern. One of the French studies described the participants as white, one study reported the participants as being Iranian, the Brazilian study reported that most participants were mixed white and black, and the trial in New Zealand reported women as being of Pacific, European or Māori descent
• Nine trials compared vitamin D to no treatment or placebo, and six trials compared vitamin D and calcium to no treatment or placebo
• Three trials included high dose supplementation arms with either 200,000 IU of vitamin D given as a single dose or 600,000 IU given twice. Daily supplementation was employed in thirteen studies, with doses ranging from 200 to 2000 IU per day. Total cumulative doses ranged from <56,000 IU in eight trials, 56,000 to 200,000 IU in five trials, to >200,000 IU in three trials
• In trials providing calcium in addition to vitamin D, calcium doses ranged from 375 mg to 600 mg of elemental calcium
• Serum 25-hydroxyvitamin D was measured by enzyme-linked immunosorbent assay (3 trials), chemiluminescent enzyme-labelled immunometric assay, liquid chromatography tandem mass spectrometry (2 trials), competitive protein binding assay (2 trials), radioligand assay, and methodology was unreported in the remaining trials

4.2 Study settings

• Bangladesh, Brazil, China, France (2 trials), India (3 trials), Iran (3 trials), New Zealand, Russian Federation, and the United Kingdom of Great Britain and Northern Ireland (2 trials)
• Trials were conducted in varying seasons, including winter-spring, summer, winter, spring-summer, and four trials were deliberately carried out in opposing seasons to balance results. Five trials did not report on season or had mixed seasons
• Thirteen studies were conducted north of the Tropic of Cancer, one was conducted between the Tropics of Cancer and Capricorn, and one was conducted on the Tropic of Capricorn

4.3 Study settings

How the data were analysed
Of the four planned comparisons, two could be performed with the available data: i) vitamin D supplementation compared to no treatment or placebo, and ii) vitamin D supplementation in combination with calcium supplementation and compared to no treatment or placebo. For trials with more than two intervention arms, groups were combined to create a single pair-wise comparison, or disaggregated for subgroup analyses. Where the control group was shared across arms, it was divided to avoid double counting of participants. Risk ratios (RR) and corresponding 95% confidence intervals (CI) were generated for dichotomous outcomes, and mean differences (MD) and 95% CI were calculated for continuous outcomes. Fixed effects meta-analysis was planned for trials examining the same intervention with similar participants, however due to substantial statistical heterogeneity, random effects meta-analysis was used. To investigate potential sources of heterogeneity, the following subgroup analyses were planned:

• By initiation of supplementation: <20 weeks’ gestation, ≥20 weeks’ gestation
• By pre-gestational body mass index (BMI): underweight (<18.5 BMI), normal weight (18.5 to 24.9 BMI), overweight (≥25 BMI), unknown/mixed
• By frequency of supplementation: single, daily, weekly
• By skin pigmentation based on Fitzpatrick skin tone chart: ≤ three, ≥ four, mixed/unknown
• By latitude: between Tropics of Cancer and Capricorn, north of the Tropic of Cancer or South of the Tropic of Capricorn
• By season at the start of pregnancy: summer, winter, unknown

Results
Vitamin D alone versus no treatment or placebo (9 trials)
Maternal primary outcomes: Pre-eclampsia, gestational diabetes, vitamin D status at term
In two studies including 219 women, the risk of pre-eclampsia was non-significantly reduced with vitamin D supplementation in comparison to no intervention or placebo (RR 0.52, 95% CI [0.25 to 1.05], p=0.069), as was the risk of gestational diabetes (RR 0.43, 95% CI [0.05 to 3.45], p=0.43). The MD in serum 25-hydroxyvitamin D at term between women treated with vitamin D versus those who received placebo or no treatment was 54.73 nmol/L (95% CI [36.60 to 72.86], p<0.00001; 7 trials/868 women). However, this finding was highly heterogeneous (I²=99%), with differences between groups in individual trials ranging from 16 to 152 nmol/L. Subgroup analysis showed a greater effect in treatment arms with a daily supplementation regimen versus those using single dosing (MD 57.80 nmol/L, 95% CI [38.37 to 77.23] and MD 15.16 nmol/L, 95% CI [5.68 to 24.63], respectively; p<0.01 for subgroup difference).

Adverse maternal effects
One trial reported on the adverse effect of maternal nephritic syndrome, in which no statistically significant difference was found between those treated with vitamin D and controls.

Additional maternal outcomes
Rates of Caesarean section were not different between intervention groups in two studies of 312 women (RR 0.95, 95% CI [0.69 to 1.31]).

Infant primary outcomes: Preterm birth (<37 weeks’ gestation), low birth weight (<2500 g)
In three trials including 477 women, the risk of preterm birth was reduced among those receiving vitamin D by 64% (RR 0.36, 95% CI [0.14 to 0.93], p=0.035). Infants born to mothers treated with vitamin D were 60% less likely to be <2500 g at birth than controls (RR 0.40, 95% CI [0.24 to 0.67], p=0.00048; 3 trials/493 participants).

Additional infant outcomes
Length at birth did not differ significantly between the treatment and control groups in pooled analysis of four trials involving 638 participants. However, in pooled analysis of data from the same studies, head circumference at birth was on average 0.43 cm greater among infants born to vitamin D supplemented mothers (95% CI [0.03 to 0.83 cm]). No differences in stillbirth and neonatal death were found in pooled analysis, although few events occurred. Apgar scores were reported in one trial of 165 women, in which no difference was found between treatment and control groups. Birth weight (g) was not found to differ between groups when considered as a continuous variable; however, when one trial with potentially incorrect reporting of birth weight data was excluded, the finding favoured the treatment group (MD 146.50 g, 95% CI [67.78 to 225.21], 4 studies/638 women). No further pre-specified outcomes were reported on in the included trials.

Vitamin D plus calcium versus no treatment or placebo (6 trials)
Maternal primary outcomes: Pre-eclampsia, gestational diabetes, vitamin D status at term
The likelihood of developing pre-eclampsia was halved with combined vitamin D and calcium treatment in comparison to no treatment or placebo (RR 0.51, 95% CI [0.32 to 0.80], p=0.0037; 3 trials/1114 women). In one trial of 54 women, gestational diabetes did not differ significantly between treatment groups (RR 0.33, 95% CI [0.01 to 7.84], p=0.50). Maternal serum 25-hydroxyvitamin D concentrations were not reported on.

Additional maternal outcomes
No significant difference between treatment groups was found in one small trial of 59 women for the outcome gestational hypertension (RR 0.26, 95% CI [0.06 to 1.12]).

Infant outcomes
In three studies involving 798 women, those receiving vitamin D plus calcium were at significantly greater risk of delivering before 37 weeks’ gestation (RR 1.57, 95% CI [1.02 to 2.43], p=0.043). The risk of neonatal death was not significantly different between treatment groups in one trial of 660 women (RR 0.20, 95% CI [0.01 to 4.15]).

Vitamin D plus calcium versus calcium
No included trials evaluated this comparison.

Vitamin D, calcium and other micronutrients versus calcium and other micronutrients
No included trials evaluated this comparison.

5. Additional author observations*

Overall, the methodological quality of the included trials was unclear, with many studies at risk of bias for allocation concealment, blinding and attrition. For the main comparison (vitamin D alone versus no intervention or placebo), the evidence for pre-eclampsia, low birth weight, and adverse effects was considered to be low quality, for gestational diabetes and maternal serum 25-hydroxyvitamin D concentrations was rated as very low quality, while the evidence for preterm births was rated as moderate quality. Although a limited number of small studies contributed to the current review and many outcomes were not reported on, 23 studies are currently ongoing or unpublished, and the results from these will greatly increase the available data for future updates of this review.

While current evidence has established that vitamin D supplementation during pregnancy increases maternal serum 25-hydroxyvitamin D concentrations at term, the clinical implications of this finding are yet to be clarified and the concentrations achieved are highly variable. Vitamin D plus calcium supplementation during pregnancy appears to reduce the risk of pre-eclampsia, while vitamin D alone appears to reduce the risk of low birth weight and preterm delivery. However, supplementation with vitamin D plus calcium appears to increase the risk of preterm birth.

Further high-quality research is required to determine whether increased maternal serum 25-hydroxyvitamin D levels translate into improved maternal and infant outcomes. In addition, research investigating which populations would attain the most benefit from antenatal vitamin D supplementation and which supplementation dose and regimen is most effective and safe is needed to inform policy makers.