Vitamin B12 deficiency and pregnancy

Seek expert advice.

Overview

• vitamin B12 (cobalamin) is an essential micronutrient critical for maternal erythropoiesis, neurological function, and fetal neural development
• physiological adaptation during pregnancy causes total serum vitamin B12 levels to fall significantly (often by ~50% by the 2nd/3rd trimester) due to hemodilution, altered carrier-protein levels (haptocorrin), and active fetal transport; total B12 alone can therefore be an unreliable indicator of true B12 deficiency
• NICE guidance (NG239) recommends active B12 (holotranscobalamin) as the preferred initial diagnostic test for suspected B12 deficiency during pregnancy
• maternal B12 deficiency is associated with megaloblastic anaemia, neurological complications, and an increased risk of congenital anomalies, specifically neural tube defects (NTDs)

Diagnostic Approach in Pregnancy (NICE NG239)

• initial testing:
  • use active B12 (holotranscobalamin) as the initial test for suspected B12 deficiency during pregnancy
  • indications to test include clinical features of deficiency (e.g., unexplained fatigue, glossitis, neurological signs, cognitive changes) or an inadequate response to iron therapy for anaemia during pregnancy or breastfeeding
• testing thresholds (NICE NG239):
  • active B12 <25 pmol/L: deficiency likely
  • active B12 25–70 pmol/L: indeterminate result; consider starting replacement if symptomatic or at high risk
  • active B12 >70 pmol/L: deficiency unlikely
• aetiology investigation:
  • if B12 deficiency is confirmed during pregnancy or breastfeeding and autoimmune gastritis is suspected, offer an anti-intrinsic factor antibody (IFA) test unless previously positive or following bariatric/GI surgery

Management and Treatment Options

Urgent Referral Indications

• seek urgent advice from a haematologist if a pregnant woman presents with confirmed or suspected B12 deficiency, particularly if accompanied by neurological symptoms or severe anaemia

Dosing Regimens

• non-dietary or irreversible causes (e.g., pernicious anaemia, bariatric/GI surgery):
  • treat with intramuscular (IM) hydroxocobalamin
  • without neurological involvement: IM hydroxocobalamin 1 mg 3 times a week for 2 weeks initially, then 1 mg every 2 to 3 months thereafter
  • with neurological involvement: seek urgent specialist advice; administer IM hydroxocobalamin 1 mg on alternate days until no further clinical improvement, followed by 1 mg every 2 months for maintenance
• dietary causes:
  • if B12 deficiency is due to dietary insufficiency (e.g., vegan diet), consider oral cyanocobalamin at a dosage of at least 1 mg daily during pregnancy or breastfeeding

Monitoring

• schedule an initial clinical review at 1 month after starting B12 treatment in pregnant or breastfeeding women (earlier than the 3-month review used for non-pregnant populations)
• B12 replacement at any stage during pregnancy is safe and is not an independent indication for additional specialized fetal monitoring

Evidence Base: Fetal Safety and Congenital Risk

Risk of Congenital Anomalies and NTDs

• periconception parental B12 status: a large prospective cohort study (Shanghai Preconception Cohort, 2026) demonstrated a significant inverse dose-response relationship between periconception parental serum B12 levels and total birth defects
  • maternal preconception B12: predicted birth defect prevalence fell from 49.6 per 1,000 pregnancies (<148 pmol/L) to 16.2 per 1,000 (>=590 pmol/L)
  • paternal preconception B12: predicted birth defect prevalence similarly decreased from 55.5 per 1,000 (<148 pmol/L) to 24.0 per 1,000 (>=590 pmol/L)
• Irish Birth Defects Study Group (Molloy et al., 2009): landmark study confirming that maternal B12 deficiency (<148 pmol/L) is an independent risk factor for neural tube defects (3- to 5-fold increased risk), independent of folate status, owing to the essential role of cobalamin in the methionine synthase pathway
  • "Deficient or inadequate maternal vitamin B12 status is associated with a significantly increased risk for neural tube defects. We suggest that women have vitamin B12 levels of >300 ng/L (221 pmol/L) before becoming pregnant."

Fetal and Neonatal Safety

• in utero exposure: there is no evidence that maternal treatment with B12 during pregnancy is associated with teratogenicity or adverse fetal effects
• trial evidence on growth: a double-blind RCT in Nepal (Chandyo et al., 2023) showed that routine maternal B12 supplementation significantly improved maternal/infant B12 biomarkers but produced no significant difference in infant growth, neurodevelopment, or haemoglobin concentrations compared to placebo

Reference:

1. National Institute for Health and Care Excellence (NICE). Vitamin B12 deficiency in over 16s: diagnosis and management. NICE Guideline [NG239]. Published March 2024.
2. NHS Specialist Pharmacy Service (SPS). Vitamin B12 deficiency: treatment during pregnancy. Published November 2024.
3. Chen H, Chen X, Zhang Y, Li M, Yao Q, Wang D, Zhao Q, et al. Periconception Parental Red Blood Cell Folate, Vitamin B12, and Birth Defects in Offspring: A Prospective Cohort Study. Annals of Internal Medicine. 2026;179(10). DOI: 10.7326/ANNALS-25-05243.
4. Molloy AM, Kirke PN, Troendle JF, Burke H, Sutton M, Brody LC, Scott JM, Mills JL. Maternal vitamin B12 status and risk of neural tube defects in a population with high neural tube defect prevalence and no folic acid fortification. Pediatrics. 2009;123(3):917-923. DOI: 10.1542/peds.2008-1173.
5. Chandyo RK, Ulak S, Kvestad I, et al. The effect of vitamin B12 supplementation during pregnancy on infant growth and development in Nepal: a community-based, double-blind, randomised, placebo-controlled trial. The Lancet Microbe / Lancet. 2023;4(4):e227-e235. DOI: 10.1016/S0140-6736(23)00346-X.