Current recommendations are for all pregnant and breast feeding women to take vitamin D supplements. The recommended dose is currently 10mcg (400IU). The reasons for this are to reduce any consequences of vitamin D deficiency, for examples rickets in children and osteomalacia in adults.
Some small studies have shown that bolus high dose vitamin D supplementation is associated with reduced risk of low birth weight. However, other studies have not demonstrated a protective effect of vitamin D supplementation on the outcome small for gestational age.
Paradoxically, a recent combined analysis of two randomised controlled trials found that higher vitamin D levels at delivery were associated with a decreased risk of comorbidities of pregnancy, including gestational diabetes, hypertension and preterm birth.
A recent systematic review and meta-analysis of observational studies found that vitamin D deficiency is associated with an increased risk of gestational diabetes, pre-eclampsia and small for gestational age babies.(BMJ 2013; 346:f1169).
If there is a causal relationship between vitamin D deficiency and adverse maternal and neonatal outcomes, then there potentially could be huge benefits from giving vitamin D supplementation to pregnant women in Asian and African countries. However, most studies so far undertaken in this area have been undertaken in developed countries.
Optimal vitamin D levels during pregnancy are still not clear. All the available evidence so far comes from observational studies, small trials, low doses of vitamin D supplementation and unclear study processes of randomisation and blinding.
So all this makes it very unclear as to whether our current recommendations are justifiable or not. Hopefully more work will be done in this area to clarify this.