Positive Spina Bifida screening

Last Thursday I had bloodwork done to check for Spina Bifida and Down Syndrome. This morning my doctor called me and said that my bloodwork came back positive for a low amount in the screening for Spina Bifida. I went in today for more blood work to check again. I'm a first time mom and feel really nervous. Neither of us have a history in our families of this. Ive taken prenatals every day for almost 2 years before actually conceiving due to infertility. Any input is appreciated. :-)