Hello Tree, so sorry to read your post. This must be a really difficult time for you.
Spina bifida causes a huge spectrum of disability, although I don't think it affects people's mental abilities, just their pelvis and legs. I wondered whether you have been offered a specialist scan at a teaching hospital? Eg in London you could ask for a second opinion scan at Great Ormond Street Hospital. Spina bifida can be very severe, but on the other hand it's thought that 5-10% of all people in the world have spina bifida occulta, which usually has no symptoms at all and is only discovered if the person has a spinal x-ray for another reason.
Here are a couple of other websites which may be useful:
www.nhs.uk/conditions/Spina-bifida/Pages/Introduction.aspx
(click on the links to other pages on the bar about 1/3 down the page)
www.asbah.org/
www.gosh.nhs.uk/gosh_families/information_sheets/neural_tube_defects_myelomeningocele/neural_tube_defects_myelomeningocele_families.html
Also, if you're deciding whether or not to continue the pregancy you can contact the charity ARC. Everyone on their telephone lines has been in a similar position to you.
www.arc-uk.org/
Finally, if you're planning a future pregnancy you'll probably want to think about taking D-chiro-inositol supplements, as well as the full dose of 4 or 5 mg folic acid, starting 3 months before the pregnancy, not just the 400 mcg everyone else takes. The Ponti study below is looking into how effective D-chiro-inositol is in preventing spina bifida in cases where a woman has had a pregnancy with spina bifida despite taking folic acid and it sounds as though you'd be eligible to take part. You're also entitled to get specialist advice about planning future pregnancies. For example, the genetics team at Great Ormond Street is happy to meet mothers who've had a spina bifida pregnancy to discuss this.
www.ucl.ac.uk/ich/research-ich/neural-development/ponti_study
Good luck over the coming weeks, and I'm sure you'll get a lot of support on Mumsnet and from your friends and family, whatever decision you make.