Meta-analyses of r and omized controlled trials ( RCTs ) document that physiological vitamin D supplementation during pregnancy is safe and improves vitamin D and calcium status , thereby protecting skeletal health .
Although certain RCTs and /or meta-analyses reported some other beneficial effects , it is still not clear whether vitamin D supplementation  improves fertility or decreases the risk of adverse pregnancy outcomes such as low birth weight , pre-eclampsia and neonatal mortality , or reduces wheeze/asthma in the infants .
Vitamin D deficiency is common and there exists a huge gap between recommended dietary vitamin D intakes and the poor vitamin D supply in the general population .
While vitamin D is important for musculoskeletal health , there are accumulating data suggesting that vitamin D may also be important for fertility , pregnancy outcomes and lactation .
Significant changes in vitamin D metabolism during pregnancy such as increased production of the " active vitamin D hormone " calcitriol support the important role of vitamin D in this setting .
Observational studies show that vitamin D deficiency is a risk marker for reduced fertility and various adverse pregnancy outcomes and is associated with a low vitamin D content of breast milk .
Nevertheless , vitamin D supplementation in pregnant women is frequently required to achieve a sufficient vitamin D status as recommended by nutritional vitamin D guidelines .
Vitamin D and risk of pregnancy related hypertensive disorders: mendelian randomisation study
Abstract Objective To use mendelian r and omisation to investigate whether 25-hydroxyvitamin D concentration has a causal effect on  gestational hypertension or pre-eclampsia .  Design One and two sample mendelian r and omisation analyses . Setting Two European pregnancy cohorts ( Avon Longitudinal Study of Parents and Children , and Generation R Study ) , and two case-control studies ( subgroup nested within the Norwegian Mother and Child Cohort Study , and the UK Genetics of Pre-eclampsia Study ) . Participants 7389 women in a one sample mendelian r and omisation analysis ( 751 with gestational hypertension and 135 with pre-eclampsia ) , and 3388 pre-eclampsia cases and 6059 controls in a two sample mendelian r and omisation analysis . Exposures Single nucleotide polymorphisms in genes associated with vitamin D synthesis ( rs10741657 and rs12785878 ) and metabolism ( rs6013897 and rs2282679 ) were used as instrumental variables . Main outcome measures Gestational hypertension and pre-eclampsia defined according to the International Society for the Study of Hypertension in Pregnancy . Results In the conventional multivariable analysis , the relative risk for pre-eclampsia was 1.03 ( 95 % confidence interval 1.00 to 1.07 ) per 10 % decrease in 25-hydroxyvitamin D level , and 2.04 ( 1.02 to 4.07 ) for 25-hydroxyvitamin D levels < 25 nmol/L compared with ≥75 nmol/L. No association was found for gestational hypertension . The one sample mendelian r and omisation analysis using the total genetic risk score as an instrument did not provide strong evidence of a linear effect of 25-hydroxyvitamin D on the risk of gestational hypertension or pre-eclampsia : odds ratio 0.90 ( 95 % confidence interval 0.78 to 1.03 ) and 1.19 ( 0.92 to 1.52 ) per 10 % decrease , respectively . The two sample mendelian r and omisation estimate gave an odds ratio for pre-eclampsia of 0.98 ( 0.89 to 1.07 ) per 10 % decrease in 25-hydroxyvitamin D level , an odds ratio of 0.96 ( 0.80 to 1.15 ) per unit increase in the log(odds ) of 25-hydroxyvitamin D level < 75 nmol/L , and an odds ratio of 0.93 ( 0.73 to 1.19 ) per unit increase in the log(odds ) of 25-hydroxyvitamin D levels < 50 nmol/L. Conclusions No strong evidence was found to support a causal effect of vitamin D status on gestational hypertension or pre-eclampsia . Future mendelian r and omisation studies with a larger number of women with pre-eclampsia or more genetic instruments that would increase the proportion of 25-hydroxyvitamin D levels explained by the instrument are needed
Meta-analyses of r and omized controlled trials ( RCTs ) document that physiological vitamin D supplementation during pregnancy is safe and improves vitamin D and calcium status , thereby protecting skeletal health .
Although certain RCTs and /or meta-analyses reported some other beneficial effects , it is still not clear whether vitamin D supplementation  improves fertility or decreases the risk of adverse pregnancy outcomes such as low birth weight , pre-eclampsia and neonatal mortality , or reduces wheeze/asthma in the infants .
Vitamin D deficiency is common and there exists a huge gap between recommended dietary vitamin D intakes and the poor vitamin D supply in the general population .
While vitamin D is important for musculoskeletal health , there are accumulating data suggesting that vitamin D may also be important for fertility , pregnancy outcomes and lactation .
Significant changes in vitamin D metabolism during pregnancy such as increased production of the " active vitamin D hormone " calcitriol support the important role of vitamin D in this setting .
Observational studies show that vitamin D deficiency is a risk marker for reduced fertility and various adverse pregnancy outcomes and is associated with a low vitamin D content of breast milk .
Nevertheless , vitamin D supplementation in pregnant women is frequently required to achieve a sufficient vitamin D status as recommended by nutritional vitamin D guidelines .