First Trimester Biochemical Screening for Trisomy 21: The Role of Free β-hCG and Pregnancy Associated Plasma Protein

Spencer K Aitken DA Crossley JA McCaw G Berry E Anderson R First trimester biochemical screening for trisomy 21: The role of free beta hCG, alpha fetoprotein and pregnancy associated plasma protein A. Ann Clin Biochem 1994; 31: 447–54.