Vitamin D in pregnancy: why your levels matter for two

Vitamin D testing rates in pregnancy have risen dramatically over the last decade in Australia — and for good reason. Pregnancy is one of the few windows in adult life where vitamin D status reliably affects two people at once, and where the consequences of deficiency are documented across multiple body systems.

Here's what the evidence shows, what the Australian guidelines recommend, and why this is one of the clearest cases in the vitamin D literature for proactive testing.

Why pregnancy increases the stakes

During pregnancy, vitamin D demand rises significantly. Maternal blood volume expands by 30–50%. The placenta is metabolically active and uses vitamin D to regulate calcium transfer to the developing baby. The fetus has no vitamin D source of its own — it's entirely dependent on the mother's circulating 25(OH)D crossing the placenta.

This means three things happen during pregnancy:

• Maternal demand goes up
• Maternal level can drop if intake doesn't keep pace
• The baby's vitamin D status is determined entirely by the mother's

What deficiency does in pregnancy

The research base on vitamin D deficiency in pregnancy is large and consistent. Documented associations include:

For the mother

• Increased risk of gestational diabetes. Several meta-analyses have shown 30–40% higher gestational diabetes rates in vitamin D deficient mothers.
• Increased risk of pre-eclampsia. The link is well-replicated, though causality is still debated.
• Higher rates of caesarean section. Possibly linked to muscle function during labour, possibly to other factors.
• Bacterial vaginosis and other infections. Modest but consistent association.

For the baby

• Lower birth weight. Severe deficiency is linked to growth restriction.
• Increased risk of neonatal rickets. Rare in modern Australia but documented, particularly in babies of mothers with darker skin tones who lived through southern Australian winters.
• Reduced bone mineralisation at birth, with effects detectable in childhood follow-up studies.
• Increased risk of childhood asthma and atopic conditions. The evidence is observational but consistent across multiple studies.
• Increased risk of childhood type 1 diabetes (in cohort studies, particularly in high-latitude populations).

None of these are inevitable consequences of mild deficiency. But the dose-response relationship is real, and addressing deficiency is one of the more cost-effective interventions in pregnancy care.

What the Australian guidelines say

The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) recommends:

• Routine vitamin D screening is not universally indicated, but should be considered for women at higher risk — darker skin tones, veiled clothing for cultural reasons, limited sun exposure, obesity, malabsorption conditions, or living in southern Australia through winter.
• Women found to be vitamin D deficient (below 50 nmol/L) should be supplemented to restore adequate levels.
• Routine vitamin D supplementation in pregnancy is not contraindicated and is widely practised even in women without documented deficiency, particularly in winter months.

The NHMRC and the Australasian Bone and Mineral Society broadly align with this approach. For the full breakdown of trimester-by-trimester targets and what to do at each stage, see our complete Australian guide to vitamin D in pregnancy.

Who's at highest risk during pregnancy

The same factors that increase vitamin D deficiency risk in the general Australian population get amplified in pregnancy:

• Women with darker skin tones living south of Brisbane — deficiency rates over 50% in some Australian pregnancy cohorts.
• Women who wear concealing clothing for cultural or religious reasons.
• Women with a high BMI at the start of pregnancy — vitamin D sequesters in adipose tissue.
• Women planning a winter birth in southern states — last trimester falls during peak deficiency season.
• Women with malabsorption conditions — coeliac, IBD, post-bariatric surgery.
• Vegan or strict vegetarian women — limited dietary vitamin D options. (See our guide for plant-based Australians.)
• Women who tested low pre-pregnancy — status doesn't typically improve through pregnancy on its own.

What testing during pregnancy looks like

Vitamin D testing in pregnancy uses the same 25(OH)D blood test as the general population. The interpretation is also the same:

• Below 50 nmol/L: deficient — supplementation indicated
• 50–75 nmol/L: sufficient but worth monitoring
• 75–125 nmol/L: optimal
• Above 250 nmol/L: excessive (rare in pregnancy)

Some clinicians prefer to maintain pregnant women in the 75–100 nmol/L range rather than the broader 50–125 range, on the basis that this gives more headroom to support the baby's needs through the third trimester when demand peaks.

Timing: a single test in the first trimester (often combined with the routine first antenatal bloods) catches deficiency early enough to act on. A repeat test in the third trimester confirms the supplementation strategy is working.

Supplementation during pregnancy

Vitamin D supplementation is considered safe and routinely recommended in Australian pregnancy care. The form, dose, and timing depend on starting status:

• Sufficient at first test (above 75 nmol/L): Maintenance dose of 1,000 IU D3 daily through pregnancy and breastfeeding, particularly through winter.
• Mildly deficient (30–50 nmol/L): 2,000–3,000 IU D3 daily, with re-test in 8–12 weeks.
• Severely deficient (below 30 nmol/L): Higher-dose protocols under GP or obstetrician supervision, sometimes including a single high-dose loading regimen, with monitored re-testing.

The standard upper safety limit during pregnancy is 4,000 IU per day without specialist supervision. Higher doses are sometimes used clinically but should not be self-prescribed.

After birth: breastfeeding

Breastfeeding doesn't replenish a baby's vitamin D meaningfully unless the mother's level is exceptionally high. Standard breast milk contains very little vitamin D. This is why the Australasian Paediatric Endocrine Group recommends:

• All breastfed babies receive 400 IU vitamin D supplementation daily for the first 12 months
• Vitamin D-deficient mothers are supplemented to support their own status and the modest milk transfer

Formula-fed babies typically don't need additional supplementation — infant formula in Australia is mandatorily vitamin D fortified.

The pragmatic strategy

For women planning a pregnancy or in early pregnancy, the practical approach is:

1. Test 25(OH)D at the first antenatal visit (or before conception if planning).
2. If deficient, start supplementation under your GP or obstetrician's guidance.
3. Re-test mid-pregnancy or late pregnancy to confirm levels are adequate.
4. Continue maintenance dose through breastfeeding.
5. Ensure baby receives daily vitamin D drops if breastfeeding.

This is one of the lowest-effort, highest-impact things you can do during pregnancy to support both your health and the baby's.

Want to test? Our at-home Vitamin D Test uses the same gold-standard LC-MS/MS method run in hospital pathology labs. Convenient when you're avoiding pathology waiting rooms or have limited time. From $49.95/kit (2-pack) with free Australia-wide shipping. Always share clinically significant results with your GP or obstetrician.