MTHFR and recurrent miscarriage: what the evidence shows

Recurrent miscarriage affects both partners. Image posed by models.

What does MTHFR actually do?

MTHFR (methylenetetrahydrofolate reductase) is an enzyme in the folate cycle. It makes the final change that turns dietary folate into 5-methyltetrahydrofolate (5-MTHF), the active form your cells use.

5-MTHF hands a methyl group to homocysteine, with vitamin B12 as the helper (the enzyme is methionine synthase). This turns homocysteine back into methionine and then into SAMe, the body's main methyl donor. SAMe supports DNA methylation, which matters a great deal in the early embryo and placenta, where cells divide rapidly and genes are being switched on and off for the first time.

The enzyme also depends on riboflavin (vitamin B2), in its active form FAD. This matters for the C677T variant, as you'll see below.

C677T and A1298C

The C677T variant, first described in 1995, makes the enzyme thermolabile (less stable at body temperature). People with two copies (TT) have higher plasma homocysteine on average Established. When folate or riboflavin intake is low, homocysteine can rise. When intake is good, the effect is often small.

The A1298C variant has a much smaller effect on homocysteine. Pooled reviews have not found a clear link between A1298C and recurrent miscarriage.

Does MTHFR cause miscarriage?

The honest answer is that it hasn't been shown to. Many case-control studies and meta-analyses have looked at it, and a 2019 overview of those meta-analyses found the results vary by population. Associations with C677T have been reported mainly in some Asian populations, and no clear association has been found for A1298C Association only.

Case-control studies like these can show an association, not cause and effect. Folate status differs between countries, and a genotype's effect depends heavily on diet. That is probably why the findings disagree.

This is why the major guidelines advise against routine MTHFR testing:

• RCOG Green-top Guideline 17 (2023), UK: routine testing for inherited thrombophilia is not recommended in recurrent miscarriage outside research, and aspirin and/or heparin should not be given for unexplained recurrent miscarriage Guideline.
• ACOG Practice Bulletin 197 (2018), USA: screening with MTHFR mutation analysis or fasting homocysteine is not recommended, because neither the heterozygous nor the homozygous C677T genotype has been associated with adverse pregnancy outcomes Guideline.
• ESHRE guideline (2018, unchanged in the 2022 update), Europe: "Measurement of homocysteine plasma levels is not routinely recommended in women with RPL". ESHRE also suggests not screening for hereditary thrombophilia unless in the context of research or in women with additional risk factors. It makes no MTHFR-specific recommendation Guideline.

Homocysteine: the part you can measure

Raised homocysteine is the biochemical route by which MTHFR could plausibly matter. A meta-analysis of case-control studies found that women with recurrent early pregnancy loss were more likely to have raised homocysteine: the pooled risk estimate was 2.7 (95% CI 1.4 to 5.2) for fasting levels and 4.2 (2.0 to 8.8) after a methionine load. The TT genotype itself gave a much weaker estimate of 1.4 (1.0 to 2.0) Association only. In other words, the biochemistry shows a stronger signal than the gene.

There are plausible ways this could affect pregnancy: raised homocysteine damages blood vessel linings, increases oxidative stress and disturbs the methylation the placenta depends on. But no trial has yet shown that lowering homocysteine prevents miscarriage Emerging.

ESHRE does not recommend measuring homocysteine routinely. I sometimes check it during preconception care anyway, because it is a cheap functional read-out of folate, B12 and B2 status. I'm always clear with patients that bringing it down is about getting the body ready for pregnancy, not a proven treatment for miscarriage.

Do I need heparin or aspirin because of MTHFR?

No, not for MTHFR alone. Two well-designed trials answer this question:

Heparin didn't help even women with established inherited clotting conditions, so there is no basis for giving it because of an MTHFR result. Antiphospholipid syndrome is different: it is an acquired condition with its own evidence and treatment, and the RCOG recommends testing for it.

Folic acid or methylfolate?

Folic acid enters the folate cycle before the MTHFR step. People with the TT genotype therefore still convert it, though less efficiently. In a large double-blind trial of 932 women of childbearing age, folic acid raised red cell folate at every dose and in every MTHFR genotype. TT carriers had lower folate concentrations than CC carriers at every dose Large RCT.

Methylfolate (5-MTHF) skips the MTHFR step altogether. One small 2024 study of 45 women with unexplained recurrent miscarriage and the TT genotype found that active folate raised red cell folate faster than folic acid. It wasn't a large randomised trial Emerging.

In practice, both forms are reasonable. What matters most is that you start at least three months before conception, at the right dose for you. Some women qualify for a higher dose than standard, and I'll check that with you.

Folic acid is taken before pregnancy mainly to prevent neural tube defects. ESHRE notes that it has not been shown to prevent miscarriage in women with unexplained recurrent pregnancy loss. Correcting folate status is part of good preparation for pregnancy, not a proven miscarriage treatment.

Riboflavin: the overlooked co-factor

MTHFR needs riboflavin to work. In a randomised trial in healthy adults, 1.6 mg of riboflavin a day for 12 weeks lowered homocysteine by 22% only in people with the TT genotype. The fall was 40% in those whose riboflavin status was low at the start. CC and CT carriers saw no change RCT, surrogate marker.

The trial wasn't in pregnancy and didn't measure miscarriage, so it shows that riboflavin corrects the biochemistry, not that it prevents loss. It is still one of the clearest examples of a genotype-specific nutrient response in this field.

What a complete assessment looks like

MTHFR should never be the whole investigation. The RCOG guideline recommends, among other things:

• testing for antiphospholipid antibodies (lupus anticoagulant and anticardiolipin antibodies) before pregnancy
• chromosome testing of pregnancy tissue from the third and later losses, and from any second-trimester loss
• assessment of the womb, ideally with 3D ultrasound
• thyroid testing.

On top of that, a preconception metabolic and nutritional assessment may include:

• serum and red cell folate, active B12 and homocysteine
• riboflavin status
• MTHFR genotype, interpreted alongside these blood results, never alone
• assessment of your partner, since sperm quality also depends on folate status.

How I work

I'm a GMC-registered GP with a special interest in methylation and MTHFR genetics. I trained in medicine at Université Paris VI and have more than 25 years of clinical experience in the UK, France, Switzerland and the USA.

I see patients in person at Hooke London, 86 Brook Street, Mayfair, and online worldwide. I work alongside your NHS, fertility or recurrent miscarriage team, not instead of them. My role is to make sure the recommended investigations have been done, and then to correct the nutritional and methylation factors that can be measured, ideally in the three months before you try to conceive.

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Frequently asked questions

Should I get tested for MTHFR after recurrent miscarriage?

Routine MTHFR testing isn't recommended by the RCOG, ACOG or ESHRE, because the result doesn't change standard treatment. If you've already been tested, the result is most useful read alongside your folate, B12 and homocysteine levels.

Does MTHFR mean I need blood thinners in pregnancy?

No. Trials have found no benefit from heparin or aspirin for unexplained recurrent miscarriage or for inherited thrombophilia. Antiphospholipid syndrome is a separate condition with its own treatment.

I have the TT genotype. Can I still take folic acid?

Yes. Folic acid still raises folate status in TT carriers. Methylfolate is a reasonable alternative. Starting at least three months before conception matters more than which form you choose.

What is a normal homocysteine level?

Reference ranges vary by laboratory. Before pregnancy I aim for the lower part of the range, but there's no trial-proven target for preventing miscarriage.

Does my partner's MTHFR status matter?

Sperm development depends on folate status, so it is worth assessing your partner's nutrition. Testing his MTHFR genotype on its own isn't recommended.

References

1. Regan L, Rai R, Saravelos S, Li TC; Royal College of Obstetricians and Gynaecologists. Recurrent Miscarriage. Green-top Guideline No. 17. BJOG. 2023. doi:10.1111/1471-0528.17515. PMID: 37334488.
2. ESHRE Guideline Group on RPL; Bender Atik R, Christiansen OB, Elson J, et al. ESHRE guideline: recurrent pregnancy loss. Hum Reprod Open. 2018;2018(2):hoy004. doi:10.1093/hropen/hoy004.
3. ESHRE Guideline Group on RPL; Bender Atik R, et al. ESHRE guideline: recurrent pregnancy loss: an update in 2022. Hum Reprod Open. 2023;2023(1):hoad002. doi:10.1093/hropen/hoad002. PMID: 36873081.
4. American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 197: Inherited thrombophilias in pregnancy. Obstet Gynecol. 2018;132(1):e18-e34. doi:10.1097/AOG.0000000000002703. PMID: 29939939.
5. Quenby S, et al. Heparin for women with recurrent miscarriage and inherited thrombophilia (ALIFE2): an international open-label, randomised controlled trial. Lancet. 2023;402(10395):54-61. doi:10.1016/S0140-6736(23)00693-1.
6. Kaandorp SP, Goddijn M, van der Post JA, et al. Aspirin plus heparin or aspirin alone in women with recurrent miscarriage. N Engl J Med. 2010;362(17):1586-96. doi:10.1056/NEJMoa1000641. PMID: 20335572.
7. Nelen WL, Blom HJ, Steegers EA, den Heijer M, Eskes TK. Hyperhomocysteinemia and recurrent early pregnancy loss: a meta-analysis. Fertil Steril. 2000;74(6):1196-9. PMID: 11119750.
8. Frosst P, Blom HJ, Milos R, et al. A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase. Nat Genet. 1995;10(1):111-3. PMID: 7647779.
9. 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. PMID: 21508090.
10. 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. PMID: 16380544.
11. Du B, Shi X, Yin C, Feng X. Polymorphisms of methalenetetrahydrofolate reductase in recurrent pregnancy loss: an overview of systematic reviews and meta-analyses. J Assist Reprod Genet. 2019;36(7):1315-28. doi:10.1007/s10815-019-01473-2. PMID: 31254142.
12. Lu Y, Hou S, Chang L, Liu P. Effect of folic acid and active folate supplementation on red blood cell folate levels in patients with MTHFR 677TT genotype and unexplained recurrent pregnancy loss [in Chinese]. J Shanghai Jiao Tong Univ (Med Sci). 2024;44(6):741-5. doi:10.3969/j.issn.1674-8115.2024.06.009.
13. NHS Tayside Clinical Genetics. MTHFR associated folate disorders. nhstayside.scot.nhs.uk.

This page is general information and does not replace personal medical advice. Last reviewed: 29 October 2026.