Arends J., Hyltoft Petersen P., and Nørgaard-Pedersen B. “6.1.2.3 Prenatal Screening for Neural Tube Defects, Quality Specification for Maternal Serum Alphafetoprotein Analysis”. Upsala Journal of Medical Sciences, vol. 98, no. 3, Sept. 1993, pp. 339-47, doi:10.3109/03009739309179330.