Pregnancy

In a successful pregnancy, maternal health is maintained, a healthy baby is delivered and the mother is able to nurture her newborn adequately.  Adequate vitamin D status is important during pregnancy to achieve optimal health. 

Metabolism 

Significant changes in maternal vitamin D and calcium metabolism occur during pregnancy, to provide the calcium needed for foetal bone mineral accretion. Approximately 25-30g of calcium is transferred to the foetal skeleton by the end of pregnancy (Specker, 2004), the majority of which is obtained in the 3rd trimester.  Calcium absorption increases from 35% in the non-pregnant state, to approximately 60% in the 3rd trimester.   and 1,25(OH)₂D increase during pregnancy.  During pregnancy, the increase in 1, 25(OH)₂ D is largely independent of PTH.

Mother-Fetus Relationship

Vitamin D can cross the placenta and most studies have found an association between maternal 25(OH)D levels and cord 25(OH)D levels, with infants typically having just 50-60% of their mother’s vitamin D stores at birth (Hollis and Pittard, 1984).   Therefore, newborns are at risk of vitamin D deficiency due to the widespread maternal deficiency.  If a mother is vitamin D replete, her infant has an 8-12 week store of vitamin D (Pettifor, 2004).  Concentrations of 25(OH)D in breast milk parallel those in the mother’s circulation (Hollis, 1984).

Status

Most studies identifying widespread vitamin D insufficiency during pregnancy in Western countries have been carried out in immigrant and dark-skinned women who are at particular risk when residing in Northern latitudes (Datta et al. 2002; Sachan et al. 2005; van der Meer et al. 2006; Maghbooli et al. 2007).  In an 'observational' study, which included 358 pregnant women (at week 12 of pregnancy living in the Hague, The Netherlands), Van der Meer (2006) observed that between 59% and 84% of these pregnant women of non-European origin had very low levels of s25(OH) D (mean (SD)15.2 (12.1) - 26.3 (25.9) nmol/L).  However, even in sunny countries such as Greece, low levels of vitamin D have been dectected in women during pregnancy.  Vitamin D deficiency during pregnancy is also prevalent in Ireland.  Preliminary data of 264 pregnant women has shown 32% of women during winter and 13% during the summer have a serum 25(OH)D level less than 25nmol/l ().  Maternal vitamin D deficiency is more likely to occur during the winter months, in countries that do not routinely fortify dairy products or other food products with vitamin D, among ethnic groups whose members cover most of their skin, and/or among individuals with heavily pigmented skin.

Deficiency

The potential long term effects of vitamin D inadequacy at the crucial stages of foetal development are unknown and are currently being investigated.  Vitamin D deficiency during pregnancy has effects on both the mother and the foetus.   Such deficiency may lead to, ,  and abnormal enamel formation in children and ,  and bone fractures in adults (Grover & Morley, 2001; Utiger, 1998; Specker, 1994). 

Recent epidemiological studies have shown an inverse relationship between weight at birth and the risk of development of specific degenerative conditions during adulthood, including obesity, , stroke, ,  and depression.  Studies have shown that infants of mothers receiving vitamin D have a greater birth weight compared to unsupplemented mothers (Mayra et al. 1981; Mayra et al. 1988).  Poor vitamin D status at birth has been associated with the development of diabetes (Hypponen et al, 2001; Hypponen et al, 2004), multiple sclerosis (Templer et al, 1992; Willer et al, 2005), and schizophrenia (McGrath et al. 2004) as well as poor bone mass (Javaid et al, 2006) in later life.

Supplementation

Vitamin D supplementation may be a solution to help alleviate the widespread occurrence of vitamin D deficiency in the general population, and specifically in pregnant women. Numerous supplementation studies have been carried out on pregnant women with positive results such as an increase in maternal 25-OH D concentration, an increase in foetus/infant 25-OHD status, increased maternal calcium concentration, reduced neonatal hypocalcemia, greater intrauterine growth, greater birth weight, greater crown-heel length, greater head circumference and improved skinfold thickness (Brooke et al, 1980; Congdon et al, 1983; Mallet et al, 1986; Mayra et al, 1981; Cockburn et al, 1980; Delvin et al, 1986).  Most of these studies have included women at high risk of vitamin D deficiency due to skin pigmentation, lack of sunlight exposure or women from countries with little or no fortification of dairy products.  All of these studies found these beneficial effects with supplementation ranging from 10µg to 30µg per day or a single oral intake of  5000µg or 15000µg (greater than vitamin D recommendations). 

In America, no national organization recommends routine vitamin D supplementation during pregnancy unless a woman is at nutritional risk ().  The UK National Institute for Clinical Excellence (NICE) on antenatal care has recommended that vitamin D supplementation should not be offered routinely to pregnant women.  Whereas, COMA, states that ‘all pregnant (and lactating) women should have an intake of 10µg of vitamin D per day. As intakes are low in these women ~3µg/d, a supplement should be recommended to pregnant and lactating women due to their higher physiological requirement  (Moy, Shaw & Mather, 2004).