In a nutshell

If your BMI (body mass index) is over 30 when you're pregnant, you'll need to take 5mg of folic acid a day - rather than the regular 400mcg dose. This will need to be prescribed by a doctor.


Ideally you should start taking it a month before you conceive, and keep taking it up to the 13th week of your pregnancy.

Even if you didn't get around to taking folic acid before you found out you were pregnant, taking it as soon as you know can still help - according to the Royal College of Obstetricians and Gynaecologists.

IMPORTANT UPDATE (Sep 2021): The UK government has announced plans to fortify non-wholemeal wheat flour with folic acid. This does not change the NHS official advice to take a folic-acid supplement daily for the 12 weeks of pregnancy and while trying to conceive.

One mum's story

As part of our Bump Project - where we got regular updates from women in different stages of their pregnancy - we heard from Milly O, who had this experience while visiting her midwife:

"My BMI was just over 30. This was higher than my booking BMI with my first child. As it had crept over 30 it meant that the strength of folic acid in standard packs of pregnancy vitamins would not be strong enough.

"I needed 5mg of folic acid instead. I will ask a bit more about it at my next appointment on Monday as I can't remember all the reasons."

What does the doctor say?

When we caught up with MFM's resident GP Dr Philippa Kaye on this one, and she said the Royal College of Obstetricians and Gynaecologists is absolutely right.

"Having a BMI of 30 or above can increase the risk of complications in pregnancy," she told MFM. "Folic acid decreases the risk of neural tube defects like spina bifida, which is an issue with the development of the spine and brain.

"Although this is rare, if your BMI is over 30 you have 3 times the risk of a neural tube defect than a woman with a BMI below 30.

"The increased folic acid dose is given to try and help decrease this risk.

"The 5mg dose is available on prescription - you would need 12.5 tablets of the 400mcg dose daily to reach 5mg!"

To calculate your BMI...

You'll need to work out your weight in kg divided by your height in metres squared. Sound a bit complicated? Don't worry - use the NHS BMI calculator for ease.

Image: Getty Images

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Tara BreathnachContent Editor and Social Media Producer

Tara is mum to 1 daughter, Bodhi Rae, and has worked as Content Editor and Social Media Producer at MadeForMums since 2015