It varies. If parents have had one child with a neural tube defect their risk of having another child with a defect is about 3 %. If they have had two children with the defect their risk increases to about 7 %. A parent who has a neural tube defect also has about a 3 % risk of having a child with a neural tube defect. All of these risks can be lowered significantly by maternal Folic Acid ingestion.
NT Recurrence rate. If you have one child with a neural tube defect, chances are higher that a second child will have one are 10-30x's that of the general population, between 1 in 40 and 1 in 100 depending on type of defect.