Folic Acid

What is it?

Folic acid is also known as vitamin B9. It is found naturally in leafy green vegetables, where it is known as folate. It is also an additive in breakfast cereals and bread.


Women and people trying to conceive, or who are in the first trimester of pregnancy, are advised to take folic acid supplements daily.

The general advice is that a dose of 400 micrograms of folic acid per day is sufficient. But the RCOG recommend that this is increased to a whopping 5 milligrams daily for “obese women” (i.e. those with a BMI greater than 30).


Folic acid supplements help the growing fetus’ spine develop properly and reduce the likelihood they will have a neural tube defect (NTD) like spina bifida.

It is very unlikely that diet alone would contain enough folic acid to prevent neural tube defects. So a supplement is recommended for all those in the first trimester of pregnancy or trying to conceive.

Data shows that ‘Overweight women’ (BMI 25.0–29.9 – see the page on BMI for explanation) are more likely – approximately 1.22 times – than ‘normal’ weight (18.5–24.9 BMI) women to have a baby with NTD. ‘Obese women’ (30.0–34.9 BMI) were 1.7 times more likely. ‘Severely obese women’ were 3.11 times more likely to have a baby with NTD.

The overall numbers are low – we’re talking about 1 in 1000 pregnancies. So remember, even if the risk has increased 3x, it’s still a very small risk. It also raises the classic ‘does correlation = causation?’ quandary.

Image shows a seagull sat on a bowed fencing panel under the caption Correlation vs Causation

We can see that being of a higher BMI is associated with greater incidence of neural tube defects. Much like in the picture above seagulls are associated with bent fences. But we don’t know if the weight/BMI is the cause of the NTD. Both conditions could be the result of a third issue – or be entirely unconnected.

Reducing Neural Tube Defects

Either way, preventing NTDs is seen as a good idea.

The RCOG, in suggesting doses of 5mg, looked at results from research on a different group of women. These women were also more likely to have a baby with NTD, but for a different reason. They had previously had a baby with a NTD, and the research found that giving them a higher dose of folic acid (4mg) daily reduced their risk of a future baby developing a NTD by 72%.

Women with a BMI of 27 or above have been found to have lower levels of folate in their blood.

Folic acid is seen as a very safe medication, with few side effects, and it’s pretty cheap. Therefore it was deemed ‘worth a try’ to see if extra folic acid could help reduce NTDs in pregnant people with higher BMIs. I mean, it makes perfect sense, right?

But does it work?

Unfortunately, a study of over 1000 women in California in 1996, recorded that while those in bigger bodies have an increased likelihood of a baby developing a NTD, taking folic acid supplements did not reduce that risk.

We have been trialling higher dose folic acid in higher BMI pregnancies in the UK for over a decade. At least, in pregnancies where the pregnant person knew about it, or the health care professionals did! Many in the Big Birthas Facebook group have reported that they’ve had to push to even get a prescription! A 2010 CMACE audit showed that just 2.1% were taking the 5mg supplement recommended.

As more research is done and more data becomes available, we will be able to see if there is any benefit.


If you would like to try higher dose folic acid to protect against NTD:

  • You could buy over-the-counter folic acid tablets which are relatively inexpensive, but they only come in 400μg tablet form. That converts to 12.5 tablets to get to the 5mg dose!
  • You can ask for a prescription from your GP. But be prepared to explain as they’ll likely be unfamiliar with the guidance!

Of course, once you are pregnant, prescriptions become free, but when trying to conceive you’ll have to pay for the prescription (unless you are exempt for other reasons).

Why not?

Reported side effects of higher doses of folic acid include nausea, abdominal distension and flatulence, allergic reactions, including skin redness, rashes, itching, hives, and shortness of breath, although these are all rare, occurring in less than 1 in a 1000 cases, and probably result from doses considerably higher than 5mg.


Risk of neural tube defect-affected pregnancies among obese women – G M Shaw et al – 1996

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