Quick answer

NIH says vitamin B12 has not been shown to cause harm, even at high doses, and sets no upper limit. A 2024 Cochrane review of 5 trials in 984 pregnant women found the evidence on pregnancy outcomes uncertain. The label of a B12 injection says well-controlled studies in pregnant women have not been done. Ask your doctor first.^1,2,3

Key facts

NIH says vitamin B12 has not been shown to cause any harm, even at high doses, and no upper limit is set because its potential for harm is low.^1,4
A 2024 Cochrane review pooled 5 trials with 984 pregnant women and found the evidence on preterm birth and low birth weight uncertain, at low certainty.²
The label of a vitamin B12 injection says adequate, well-controlled studies have not been done in pregnant women, and that needs rise in pregnancy.³
NIH says blood B12 levels tend to drop during pregnancy, sometimes to subnormal levels, and usually return to normal after delivery.⁴
NIH says infants of women who follow vegan diets can develop B12 deficiency very early in life, and that it can be severe.⁴

What does NIH say about vitamin B12 in pregnancy?

NIH's fact sheet for consumers answers the harm question first. It says vitamin B12 has not been shown to cause any harm, even at high doses, and the professional sheet explains that the Food and Nutrition Board set no upper limit because B12 has a low potential for toxicity and the body does not store the excess. Neither sheet gives a separate limit for pregnancy.^1,4

The professional sheet lists a higher recommended intake in pregnancy than for other adults. It also describes two points that matter for pregnancy and early infancy.⁴

Blood B12 levels tend to fall during pregnancy, sometimes below normal, and usually return to normal after delivery.⁴

Breastfed babies of women who eat no animal products can have very limited B12 reserves, because little B12 reaches them through the placenta and breast milk.⁴

What did the Cochrane review of pregnancy trials find?

The 2024 Cochrane review looked at oral vitamin B12 taken during pregnancy compared with placebo or nothing. It found 5 trials with 984 women, all in lower- and middle-income countries, where between 26% and 51% of the women were B12 deficient at the start. Amounts in the trials ranged widely, and supplements began between 8 and 28 weeks of pregnancy.²

Selected results from the 2024 Cochrane review of vitamin B12 in pregnancy²
Outcome	What the review found	Certainty
Maternal B12 deficiency	25.9% with B12 and 67.9% without (2 trials, 272 women)	Very low
Preterm birth	No clear difference (risk ratio 0.97, range 0.55 to 1.74; 2 trials, 340 women)	Low
Low birth weight	Uncertain (risk ratio 1.50, range 0.93 to 2.43; 2 trials, 344 women)	Low
Miscarriage	Reported in 2 trials, but not in a form that could be pooled	Not rated
Neural tube defects	No trial examined them	None

The authors concluded that B12 supplements may raise maternal and infant B12 levels but that the effect on longer-term health for mothers and children has not yet been established. In plain terms, the sources show that B12 levels rise with supplements and say little either way about pregnancy outcomes.²

What does a vitamin B12 injection label say about pregnancy?

A vitamin B12 injection is a prescription product used when the body cannot absorb B12, but its label addresses pregnancy. It says adequate and well-controlled studies have not been done in pregnant women, that vitamin B12 is an essential vitamin whose requirements are increased during pregnancy, and that increased needs in pregnancy can usually be met with oral supplementation.³

The injection label also lists rare but serious reactions, including anaphylaxis, which applies to injected B12 and not to oral supplements. The question of whether injections are right during pregnancy is one for the prescriber who is managing the deficiency.³

Who should ask a doctor before taking vitamin B12 in pregnancy?

Anyone who is pregnant should ask a doctor or midwife before adding a supplement, and NIH points to some people for whom B12 deserves a conversation.⁴

Anyone who eats no animal foods, since B12 occurs naturally only in animal foods and infants of vegan mothers can develop deficiency.^1,4
Anyone with pernicious anemia, a stomach or bowel condition or past stomach surgery, who may need injections or higher doses under medical care.⁴
Anyone taking metformin or a proton pump inhibitor such as omeprazole, which can lower B12 levels over time.⁴
Anyone with numbness or tingling in the hands or feet, or balance problems, which need testing and not self-treatment.⁴
Anyone taking large amounts of folic acid, which can hide low B12.^1,4

Questions people ask

Is vitamin B12 ok during pregnancy?

NIH says B12 has not been shown to cause harm even at high doses, and a 2024 Cochrane review found its effects on pregnancy outcomes uncertain. Ask your doctor, especially if you eat no animal foods.^1,2

Are vitamin B12 injections safe during pregnancy?

The label of a B12 injection says well-controlled studies have not been done in pregnant women and that needs in pregnancy can usually be met with oral supplementation. It also lists rare serious reactions. Ask the prescriber who manages your B12.³

Is vitamin B12 supplement safe during pregnancy?

NIH sets no upper limit for B12 and says it has not been shown to cause harm, but also says blood levels tend to dip in pregnancy and return to normal afterward. Ask your doctor whether a supplement fits your diet.^1,4

Is vitamin B12 safe in the first trimester?

The sources checked do not give a separate caution for the first trimester. The 2024 Cochrane review's trials started B12 between 8 and 28 weeks, and none reported on neural tube defects. Ask your doctor.²

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.