Vitamin B12 and birth defects: the father's level counts too

Most preconception advice about a single nutrient is aimed at the mother. A large new cohort study puts a number on why the father's status may belong in that conversation too.

Researchers following couples in China found that low vitamin B12 in either parent before conception was linked to a higher rate of birth defects in their children, with a clear dose-response pattern: the more B12 in the blood, the lower the predicted rate, for mothers and, to a smaller degree, for fathers.

The study

The findings come from the Shanghai Preconception Cohort, published online September 1, 2026 in Annals of Internal Medicine, led by researchers at the Children's Hospital of Fudan University. It combined two groups: 3,032 couples who had blood drawn within four months before conception (73 birth defects among their children), and a larger group of 17,765 women whose blood was drawn at four months of pregnancy or earlier (327 birth defects). Birth defects were counted across live births, stillbirths and pregnancies ending because of a diagnosed fetal abnormality.

For vitamin B12, the pattern was consistent and graded. Predicted birth defect prevalence fell from 49.6 to 16.2 cases per 1,000 pregnancies as preconception maternal B12 rose from the lowest category (below 148 pmol/L) to the highest (590 pmol/L or above). A similar but weaker pattern held for fathers: 55.5 to 24.0 cases per 1,000, with the association flattening out at higher paternal levels.

Folate was messier. In the same preconception couples, the folate pattern was less consistent for both parents. But in the larger group of women measured in early pregnancy, higher red blood cell folate was linked to fewer birth defects too: 25.7 cases per 1,000 pregnancies below 453 nmol/L, versus 16.5 per 1,000 at 906 to 1,131 nmol/L.

This is an observational cohort, not a trial, and the authors flag residual confounding as a limitation: couples with better nutrient status likely differ from those without it in other ways diet and blood tests do not capture. An editorial accompanying the paper, from researchers at the National University of Singapore, pointed to animal research showing that folate insufficiency in males can alter the epigenetic marks carried in sperm, a possible mechanism for a paternal effect. That link has not been shown directly in humans, and the editorial frames it as a hypothesis rather than a settled explanation.

Why the father's number is the news

Folate fortification and prenatal vitamins have spent decades built around one message: get the mother's folate up before conception. This dataset does not overturn that message, folate before conception still tracks with the outcome, just less cleanly than B12 does here, but it adds a second one: a father's B12 status, measured before conception, tracked with the same outcome in his partner's pregnancy. That is a nutrient most preconception counseling never asks a man about.

Getting enough vitamin B12

Vitamin B12 occurs naturally only in animal foods, which is the reason it is the nutrient vegans are told to watch most closely. Amounts below are per realistic serving, ordered by density:

• Beef liver, cooked, 3 oz (85 g): 70.7 mcg
• Mussels, steamed, 3 oz (85 g): roughly 20 mcg
• Clams, cooked, 3 oz (85 g): 17 mcg
• Oysters, cooked, 3 oz (85 g): 14.9 mcg
• Nutritional yeast, fortified, ¼ cup: 8 to 24 mcg, check the label, fortification varies a great deal by brand
• Alaska king crab, steamed, 3 oz (85 g): roughly 10 mcg
• Sardines, canned, 1 can (about 3.75 oz): roughly 9 mcg
• Rainbow trout, cooked, 3 oz (85 g): roughly 5 mcg
• Salmon, cooked, 3 oz (85 g): 2.6 mcg
• Tuna, canned, 3 oz (85 g): 2.5 mcg
• Ground beef, cooked, 3 oz (85 g): 2.4 mcg
• Milk, 1 cup (240 ml): 1.3 mcg
• Plain yogurt, 6 oz: 1.0 mcg
• Breakfast cereal, fortified with 25% of the Daily Value, 1 serving: 0.6 mcg
• Ham, cured and roasted, 3 oz (85 g): 0.6 mcg
• Egg, whole, hard-boiled, 1 large: 0.6 mcg
• Chicken breast, roasted, 3 oz (85 g): 0.3 mcg

How much you need

The US RDA for vitamin B12, from the National Academies:

• Infants 0–6 months (AI): 0.4 mcg
• Infants 7–12 months (AI): 0.5 mcg
• Children 1–3 years: 0.9 mcg
• Children 4–8 years: 1.2 mcg
• Children 9–13 years: 1.8 mcg
• Teens and adults, 14 and up, both sexes: 2.4 mcg
• Pregnancy: 2.6 mcg
• Breastfeeding: 2.8 mcg

No tolerable upper intake level has been set. B12 is water-soluble, is not stored in the body in meaningful excess, and the NIH Office of Dietary Supplements reports no confirmed harm from high intake through food or supplements.

Too little, and too much

Deficiency builds slowly and shows up as fatigue and weakness, pale skin, a smooth or sore tongue, numbness or tingling in the hands and feet, balance problems, and memory or mood changes. The blood picture is megaloblastic anemia. Left uncorrected for long enough, the nerve damage can become permanent.

Risk is concentrated in a few groups: anyone eating vegan or largely plant-based, since the vitamin is not in plant foods without fortification; adults over 50, because reduced stomach acid makes it harder to release B12 from food protein, a mechanism that pushes deficiency prevalence from roughly 3–5% in the general population to 5–20% in older adults; long-term users of metformin or of acid-reducing drugs like proton pump inhibitors and H2 blockers, both of which interfere with absorption; and anyone with pernicious anemia or a history of gastric or bariatric surgery.

Too much is not really a documented problem for B12 specifically, which is why there is no upper limit on the label. That is not true of every nutrient in a prenatal formula, and it is not a reason to supplement without a reason.

Tracking it in AIM

AIM does not track vitamin B12 as standard, so add it as a custom metric: give it a name, a unit (mcg) and a daily target of 2.4 mcg (2.6 mcg during pregnancy, 2.8 mcg while breastfeeding), and it is estimated on every meal from then on, pinned to your cards or left behind "more".

Estimates from a photograph carry real uncertainty for micronutrients, because content varies with soil, cultivar, storage and cooking in ways no image contains. Treat the week as meaningful and any single day as approximate.