To say that no spinal tissue is involved is good but there can be no certainty about it. No ultrasound examination is 100% correct (no matter what you may believe). Your anxiety of a subsequent involvement is justified. Surgical corrections may be complete but in a significant number some handicap may be left. Please take a decision based on all this. No body will be able to give you a guarantee that the baby is absolutely ok. You can write again if you need any more clarification. Even if the baby is ok you may need to rule out chromosomal anomalies with a fetal blood sample. I know that it is a difficult decision but you have to make a choice between a poor decision and a terrible one. Please choose the least terrible of the choices and if you have normal children and would not like to continue this pregnancy for fear of life long handicap I would agree with you.