Normal posterior fossa = no spina bifida?

On the first Friday of every month, I attend the Fetal Boards meeting at our hospital.  It's actually intended for the residents, but I like to sit in on them because it's a great way to learn what the doctors do with the information we give them from an ultrasound.  A few months ago, the meeting was on spinal anomalies.  I found this meeting particularly interesting because I am always worried about missing a neural tube defect.  After the meeting, I asked the doctor, "If the cerebellum and cisterna magna appear normal, what are the chances that the fetus will have a spina bifida?"  His reply was "None."  Now, I have learned in this profession that when it comes to fetal development, never say never.  However, it is reassuring to hear that even if I cannot get perfect views of the spine, a normal posterior fossa of the brain will virtually eliminate the chance of missing a spina bifida.