B12 for plant-based diets.
Vitamin B12 is found almost exclusively in animal products. If you follow a vegetarian or vegan diet, understanding B12 is critical for your pregnancy and your baby's development. Deficiency is common and the consequences during pregnancy are serious, but entirely preventable with appropriate supplementation.
Why it matters
Vitamin B12 is found almost exclusively in animal products. If you follow a vegetarian or vegan diet, understanding B12 is critical for your pregnancy and your baby’s development.
If you follow a vegetarian or vegan diet, B12 supplementation is essential before pregnancy, throughout pregnancy, and throughout breastfeeding. This is not a lifestyle choice debate. It is a safety issue.
B12 deficiency increases neural tube defect risk 2-3 fold, independently of folate status. Folate supplements do not protect against B12-related harm.
Liver stores of B12 total 2-5 mg. With zero intake, clinical deficiency may take 2-5 years to appear, but functional deficiency (elevated MMA and homocysteine) can develop within months.
Have your B12 levels tested preconception or in early pregnancy if you eat little or no animal products. Serum B12 testing is covered under Medicare when clinically indicated.
The numbers
Nutrients 2024, 16(10):1442
ABS 2023
Niklewicz et al. 2024, Nutrition Bulletin
Niklewicz et al. 2024
Neural tube defect risk
B12 deficiency independently increases neural tube defect risk by 2-3 fold
Molloy et al. (2009, Pediatrics) studied 278 NTD cases across three nested case-control studies. Women in the lowest B12 quartiles had 2-3 times higher adjusted odds of an NTD-affected pregnancy. Women with B12 below 150 ng/L had roughly 5 times higher risk compared to those above 400 ng/L. These effects persisted after adjusting for folate status, establishing B12 as an independent risk factor.
Rogne et al. (2017, Am J Epidemiol) conducted an individual participant data meta-analysis of 22 studies (n=11,993) and found B12 deficiency was associated with 15% higher risk of low birth weight (RR 1.15, 95% CI 1.01-1.31) and a linear association with preterm birth.
A Nepal RCT of 50 mcg B12 from early pregnancy through 6 months postpartum improved infant growth and Bayley-III developmental scores.
Molloy et al. 2009, Pediatrics (PMID: 19255021); Rogne et al. 2017 (DOI: 10.1093/aje/kww212); 2025 meta-analysis, Nutrients 17(12):2040, 38 studies, 2,316 NTD cases
Safety
There are several important safety considerations for plant-based women during pregnancy. These are not meant to cause alarm, but to ensure you have the information you need.
Folic acid corrects the anaemia caused by B12 deficiency but does NOT correct the neurological damage. B12 is required to convert 5-methylTHF to THF. Without B12, folate becomes metabolically trapped. Supplemental folic acid bypasses this at the blood cell level but not the nerve level. Australian pregnant women routinely take 400-5,000 mcg folic acid daily. If a vegetarian or vegan woman is simultaneously B12-deficient, folate supplementation may mask the anaemia while neurological damage progresses silently in both mother and fetus. Reynolds (2019, BMJ) reported "consistent evidence that excess folate in the presence of B12 deficiency can be directly harmful to the nervous system." A mouse model (Zarbalis et al., 2023) found the combination caused more severe neuronal damage than either condition alone.
Reynolds 2019, BMJ (PMC6889897); Zarbalis et al. 2023, Communications Biology
The predominant cobamide in spirulina is pseudovitamin B12 (83%), which is not bioavailable. Pseudovitamin B12 from spirulina and most algae is not usable by the human body and may actually compete with and inhibit absorption of true B12, potentially worsening deficiency. Nori seaweed contains some true B12 but in unreliable amounts. No plant source provides adequate B12 for pregnancy.
Watanabe et al. 1999, J Agric Food Chem (PMID: 10552882)
Maternal B12 status directly determines breastmilk B12 content. Severe infant neurological damage has been documented when vegan mothers stopped supplementing after birth. Since at least 1980, over 30 published case reports document severe B12 deficiency in exclusively breastfed infants of unsupplemented vegan or vegetarian mothers. Symptoms can appear as early as 2 weeks of age in severely depleted mothers, though more commonly at 4-12 months. One Italian case: the vegan mother supplemented B12 during pregnancy but stopped during lactation, resulting in her 5-month-old presenting with haemoglobin of 4.7 g/dL and pancytopenia. The NHMRC states: "To ensure adequate vitamin B12 status in their infants, vegan mothers should supplement their diets with vitamin B12 at the RDI level throughout pregnancy and lactation."
Grattan-Smith et al. 1997; von Schenck 1997; Esposito et al. 2012, BMC Pediatrics; NHMRC
Breastfeeding
Maternal B12 status directly determines breastmilk B12 content. Over 30 published case reports document severe B12 deficiency in exclusively breastfed infants of unsupplemented vegan or vegetarian mothers. This is entirely preventable.
Irritability, poor feeding, failure to thrive, developmental delay or regression. In severe cases, seizures and lethargy.
Grattan-Smith et al. 1997 (Australian cases)
Three Australian infants presented with seizures, developmental regression, and one comatose with brain atrophy on MRI. Of 7 vegan infants followed long-term (up to 12 years), 5 had abnormal neurological development at final follow-up.
Grattan-Smith et al. 1997; von Schenck 1997 (25 cases)
All documented cases of infant B12 deficiency from breastfeeding occurred in mothers who were not adequately supplementing. With appropriate B12 supplementation throughout lactation, this risk is eliminated.
NHMRC, Vegan Society guidelines
Recommended intake by stage
Preconception
2.4 mcg/day (RDI), or 10 mcg daily / 2,000 mcg weekly for vegans
Build stores before pregnancy. Only newly absorbed B12 crosses the placenta, so adequate ongoing intake matters more than large stores.
Testing: Test serum B12 (and HoloTC if available) before conceiving. If deficient, start 1,000 mcg/day oral for repletion.
Pregnancy
2.6 mcg/day (RDI), higher if previously deficient
B12 requirements increase modestly in pregnancy. The folate-B12 masking trap means deficiency can progress silently while taking prenatal vitamins. Direct B12 testing (not reliance on blood count or MCV) is essential in at-risk women.
Testing: Test in first trimester if not tested preconception. Repeat if initially borderline.
Breastfeeding
2.8 mcg/day (RDI), or 10 mcg daily / 2,000 mcg weekly for vegans
Maternal B12 status directly determines breastmilk B12 content. Do not stop supplementation after birth. Over 30 case reports document severe infant neurological damage from unsupplemented vegan breastfeeding mothers.
Testing: Monitor if previously deficient. Watch for infant signs: irritability, poor feeding, developmental regression.
Supplements and testing
Despite marketing claims, dose and regularity matter far more than which form of B12 you take. Here is what the evidence shows about different supplement forms and testing.
Supplement forms compared
Most studied form. Widely available, stable, inexpensive. Most clinical trials used this form.
A direct comparison in vegans actually found cyanocobalamin yielded better holotranscobalamin levels (median 150 vs 78.5 pcg/L). The NIH states: "No evidence indicates that absorption rates vary by form."
Marketed as the "active form." Less stable (light and heat sensitive). More expensive.
Despite marketing claims, clinical evidence does not support its superiority. All cobalamin forms are degraded to base cobalamin upon absorption and reconverted endogenously. Obeid, Fedosov, and Nexo (2015) concluded "cobalamin coenzyme forms are not likely to be superior."
Injectable form preferred for severe deficiency or neurological involvement. Available by prescription in Australia.
Preferred for repletion of severe deficiency due to higher retention. Intramuscular 1,000 mcg doses used for acute correction.
Testing
Standard first-line test. Declines physiologically in pregnancy and can appear low without true tissue deficiency. Covered under Medicare when clinically indicated.
Limitation: Can be misleadingly low in pregnancy due to haemodilution
Most sensitive early marker of B12 depletion. Increasingly available through Australian pathology labs, often auto-triggered when serum B12 is borderline.
Limitation: Not universally available as a stand-alone order
Specific to B12 (unlike homocysteine, which rises in both B12 and folate deficiency). Elevated MMA confirms tissue-level B12 insufficiency.
Limitation: May be less useful in pregnancy specifically due to physiological changes
Who should be tested?
- Women following vegetarian or vegan diets
- Women with malabsorption disorders (coeliac disease, Crohn's)
- Women taking metformin (depletes B12 over time)
- Women who have had bariatric surgery
- Women with unexplained macrocytic anaemia or neurological symptoms
Sources
All recommendations in this guide are drawn from peer-reviewed research, clinical guidelines, and systematic reviews. Key sources are listed below.
Sources: Niklewicz et al. 2024 (Nutrition Bulletin, PMID: 39373282), Molloy et al. 2009 (Pediatrics, PMID: 19255021), Rogne et al. 2017 (Am J Epidemiol, DOI: 10.1093/aje/kww212), Reynolds 2019 (BMJ, PMC6889897), Zarbalis et al. 2023 (Communications Biology), Grattan-Smith et al. 1997, von Schenck 1997, Esposito et al. 2012 (BMC Pediatrics), NHMRC Nutrient Reference Values, RANZCOG C-Obs 25, RACGP 2019, ABS 2023, Watanabe et al. 1999 (PMID: 10552882), Obeid et al. 2015 (Mol Nutr Food Res)
herjourney.com.au · Health education, not medical advice
Evidence: Niklewicz 2024, Molloy 2009, Rogne 2017, NHMRC, RANZCOG C-Obs 25 · last updated March 2026 · © Her Journey 2026