MTHFR and pregnancy: don’t forget your B12
MTHFR and pregnancy: don’t forget your B12
If a gene test has told you that you carry an MTHFR variant and you’re planning a pregnancy, most of what you’ll read online is about which form of folate to take. Folate does matter. Taken before conception, it prevents most neural tube defects, such as spina bifida.[1][2] An MTHFR result means choosing your folate with care. It never means going without.
What gets missed is vitamin B12. Folate and B12 work as a pair, and folate can’t do its job when B12 is low. Women with the lowest B12 in early pregnancy had two to three times the odds of a neural tube defect,[3] and in one Indian study, children of mothers with high folate but low B12 were the most insulin resistant by age six.[4]
A 2026 review in Frontiers in Nutrition brought this together. It found that folate’s effects on pregnancy outcomes depend on timing, amount, duration, MTHFR genetics and B12 status, and that no ideal blood folate range has been agreed yet.[5]
The gene result is a starting point. Your folate, active B12 and homocysteine levels show where you actually stand, and because they’re rarely checked in routine pregnancy care,[6] you usually have to ask. A few months before you start trying is the right time.
What an MTHFR result really tells you
MTHFR is an enzyme. Its job is to convert folate into its active form, methylfolate, which your body uses to build DNA and to recycle homocysteine. The gene behind it has a common variant called C677T. One copy (CT) slows the enzyme a little. Two copies (TT) slow it more.
How common TT is depends on ancestry, from about 1% to 20% or more. A CDC review found a moderately higher risk of spina bifida when the mother carried TT, a risk that seemed to depend on her folate status.[9] Pooling 40 studies, women with TT had blood folate about 13 to 16% lower than women with CC.[7] In a large supplementation trial, the TT group ran lower folate at every amount tested.[10]
The variant is real. What it can’t tell you is where your levels sit today. The American College of Medical Genetics advises against routine MTHFR testing, including after pregnancy loss, because a genotype on its own rarely changes care.[8] Your actual numbers do. For how MTHFR fits the bigger picture, see our guide to the wider folate cycle.
It’s your folate status that counts
Two landmark trials in the early 1990s showed that folate supplements taken around conception prevent most neural tube defects. In women who had already had an affected pregnancy, the repeat rate fell by 72%.[1] In women planning a first pregnancy, a multivitamin containing folate brought cases down to zero, against six in the comparison group.[2]
What protects the baby is the folate that reaches your cells. CDC modelling found neural tube defect risk keeps falling as red cell folate rises, only levelling off at higher levels.[11] So it pays to measure rather than guess.
Many people with an MTHFR variant prefer methylfolate, because it skips the step the enzyme struggles with. In a 24-week randomised trial, methylfolate raised red cell folate more than the same amount of the standard form, and levels were still climbing when the study ended.[12] Folate stores take months to build, which is one more reason not to leave it late.
Whichever form you use, the rule doesn’t change. Keep folate going before and during early pregnancy, and decide the form and amount with your practitioner.
B12, the half of the pair that gets forgotten
Folate and B12 meet at one enzyme, methionine synthase. It needs both to recycle homocysteine and keep the methylation cycle turning. When B12 runs low, folate gets stuck in a form your cells can’t use. Blood folate can look fine while the cells go short.
I see this often in clinic. A woman has read about MTHFR, switched to methylfolate and done everything right on folate, and nobody has checked her B12. In three groups of Irish women, those with the lowest B12 in early pregnancy had two to three times the odds of a neural tube defect compared with those with the highest.[3] The lead researcher has since argued that B12 may protect in its own right.[13]
The balance between the two seems to matter too. In the Pune Maternal Nutrition Study, two thirds of mothers had low B12 but almost none had low folate. By age six, the children whose mothers combined high folate with low B12 were the most insulin resistant.[4] That’s an association, not proof. It’s still a good reason not to push folate while ignoring B12.
The encouraging part: in a 2026 trial in mostly vegetarian pregnant women, a higher B12 supplement lifted mothers’ B12 status and their babies’ mental development scores at around one year, compared with a lower one.[14]
B12 also tends to drift down as pregnancy goes on,[6] so starting low leaves little margin. Those most at risk include vegetarians and vegans, people on metformin or long-term acid blockers, and anyone with coeliac disease, low stomach acid or past weight-loss surgery. Standard serum B12 can look normal while cells are short, which is why we test active B12. More on that in our article on hidden B12 deficiency.
Gene result or status marker?
Your genotype never changes. These markers show what is happening in your body now.
| Test | What it shows | Before pregnancy |
|---|---|---|
| MTHFR genotype | Whether you carry the C677T variant, and how many copies | A nudge to look closer. It never changes and says nothing about your levels. |
| Folate | Your current folate status | Shows whether you’re starting from a good level, and whether your folate is doing its job. |
| Total B12 | All B12 in the blood, active and inactive | Can sit in range while cells go short. |
| Active B12 | The share of B12 your cells can actually use | An earlier, more useful sign of low B12. |
| Homocysteine | How well folate, B12 and B6 are recycling it | Rises when the folate and B12 pair is struggling. |
In a 2024 Spanish study of 831 pregnancies, nearly half of women had suboptimal red cell folate at 12 weeks, and only about a third had started supplements before conceiving.[6]
Homocysteine: where genes and nutrients meet
Homocysteine rises whenever the folate and B12 pair is under strain, which makes it a handy summary marker. In a Norwegian study covering more than 14,000 pregnancies, women in the highest quarter of homocysteine had a 32% higher risk of preeclampsia, a 38% higher risk of premature birth and about twice the risk of very low birth weight, compared with the lowest quarter.[15] These are associations, measured years after the pregnancies, but they fit the biology.
Homocysteine also shows how much a gene result depends on nutrition. The MTHFR enzyme needs riboflavin (vitamin B2) to work. In a randomised trial in adults, extra riboflavin lowered homocysteine only in people with TT, and most of all in those who started low in riboflavin.[16] The variant mattered less once the enzyme had what it needed.
What the 2026 review adds
Yang and Li looked past neural tube defects to gestational diabetes, blood pressure disorders, fetal growth, miscarriage and preterm birth.[5] The neural tube benefit is settled. For the other outcomes, results are mixed: the effect seems to depend on when supplements start, how much is taken and for how long, and some studies show U-shaped patterns where both low and high intakes are linked with problems. MTHFR genetics and B12 status both shift how folate behaves, and nobody has yet agreed on an ideal blood folate range.
The authors’ answer is precision nutrition, planning around a woman’s own folate levels, genetics and B12 status. In clinic, we call it testing before guessing.
A precision plan before you conceive
Start about three months before you begin trying, roughly how long your folate stores take to build.
Keep folate going. Your genotype shapes which form you choose. It doesn’t change whether you need it.
Test more than the gene. Ask for folate, active B12 and homocysteine.
Sort B12 early, especially if you’re vegetarian, take metformin or acid blockers, or have gut absorption issues.
Choose the form and amount together. Dosage is set by individual assessment and testing, not by a genotype.
Recheck. B12 in particular tends to fall as pregnancy goes on.
This is a regular part of our MTHFR and methylation care, using functional testing to show where your levels sit. Your GP and obstetrician stay central to your pregnancy care. The goal is to walk into those appointments with a clearer picture.
Key Insights
Frequently Asked Questions
Should I stop taking folate if I have an MTHFR variant?
No. Folate before conception prevents most neural tube defects, whatever your genotype. An MTHFR variant means your folate may run lower, so check your levels and choose the form with care. Many people with MTHFR prefer methylfolate. Decide the form and amount with your practitioner, based on testing.
Do I need an MTHFR test before getting pregnant?
Usually not. Medical genetics guidelines advise against routine MTHFR testing because a genotype on its own rarely changes care. Folate, active B12 and homocysteine levels are more useful because they show your actual status. If you already know your genotype, it helps interpret those results.
Why does vitamin B12 matter so much in pregnancy?
B12 and folate work together to recycle homocysteine and build DNA. When B12 is low, folate cannot do its job properly. Low B12 in early pregnancy has been linked to a higher risk of neural tube defects, and high folate with low B12 has been linked to insulin resistance in children. B12 also tends to fall during pregnancy, so starting with good levels matters.
When should I start preparing if I want to get pregnant?
Around three months before you start trying. Folate stores take months to build, and low B12 can take time to correct. Testing early leaves room to adjust and recheck before conception, rather than finding a gap in the first trimester.
Ready to find answers?
Planning a pregnancy and not sure where your folate and B12 actually sit? We can test your levels and plan around them, alongside your GP and obstetrician.
References
- MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. Lancet. 1991;338(8760):131–7. PMID: 1677062
- Czeizel AE, Dudás I. Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation. N Engl J Med. 1992;327(26):1832–5. doi:10.1056/NEJM199212243272602
- Molloy AM, Kirke PN, Troendle JF, et al. 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):917–23. doi:10.1542/peds.2008-1173
- Yajnik CS, Deshpande SS, Jackson AA, et al. Vitamin B12 and folate concentrations during pregnancy and insulin resistance in the offspring: the Pune Maternal Nutrition Study. Diabetologia. 2008;51(1):29–38. doi:10.1007/s00125-007-0793-y
- Yang R, Li G. The roles of folate, MTHFR genetics, vitamin B12 in pregnancy outcomes. Front Nutr. 2026;13:1785263. doi:10.3389/fnut.2026.1785263
- Santos-Calderón LA, Cavallé-Busquets P, Ramos-Rodríguez C, et al. Folate and cobalamin status, indicators, modulators, interactions, and reference ranges from early pregnancy until birth: the Reus-Tarragona birth cohort study. Am J Clin Nutr. 2024;120(5):1269–83. doi:10.1016/j.ajcnut.2024.09.015
- Tsang BL, Devine OJ, Cordero AM, et al. Assessing the association between the methylenetetrahydrofolate reductase (MTHFR) 677C>T polymorphism and blood folate concentrations: a systematic review and meta-analysis of trials and observational studies. Am J Clin Nutr. 2015;101(6):1286–94. doi:10.3945/ajcn.114.099994
- Hickey SE, Curry CJ, Toriello HV. ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing. Genet Med. 2013;15(2):153–6. doi:10.1038/gim.2012.165
- Botto LD, Yang Q. 5,10-Methylenetetrahydrofolate reductase gene variants and congenital anomalies: a HuGE review. Am J Epidemiol. 2000;151(9):862–77. doi:10.1093/oxfordjournals.aje.a010290
- Crider KS, Zhu JH, Hao L, et al. MTHFR 677C→T genotype is associated with folate and homocysteine concentrations in a large, population-based, double-blind trial of folic acid supplementation. Am J Clin Nutr. 2011;93(6):1365–72. doi:10.3945/ajcn.110.004671
- Crider KS, Devine O, Hao L, et al. Population red blood cell folate concentrations for prevention of neural tube defects: Bayesian model. BMJ. 2014;349:g4554. doi:10.1136/bmj.g4554
- Lamers Y, Prinz-Langenohl R, Brämswig S, Pietrzik K. Red blood cell folate concentrations increase more after supplementation with [6S]-5-methyltetrahydrofolate than with folic acid in women of childbearing age. Am J Clin Nutr. 2006;84(1):156–61. doi:10.1093/ajcn/84.1.156
- Molloy AM. Should vitamin B12 status be considered in assessing risk of neural tube defects? Ann N Y Acad Sci. 2018;1414(1):109–25. doi:10.1111/nyas.13574
- Nagpal J, Mathur M, Rawat S, et al. Maternal supplementation of vitamin B12 in predominantly vegetarian pregnant women improves their vitamin B12 status and the neurodevelopment of their infants: the MATCOBIND multicentric double-blind randomised control trial. BMJ Paediatr Open. 2026;10(1). doi:10.1136/bmjpo-2025-004112
- Vollset SE, Refsum H, Irgens LM, et al. Plasma total homocysteine, pregnancy complications, and adverse pregnancy outcomes: the Hordaland Homocysteine study. Am J Clin Nutr. 2000;71(4):962–8. doi:10.1093/ajcn/71.4.962
- McNulty H, Dowey LRC, Strain JJ, et al. Riboflavin lowers homocysteine in individuals homozygous for the MTHFR 677C→T polymorphism. Circulation. 2006;113(1):74–80. doi:10.1161/CIRCULATIONAHA.105.580332
