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Title
Re: First-trimester combined screening for trisomy 21 with different combinations of placental growth factor, free β-human chorionic gonadotropin and pregnancy-associated plasma protein-A. K. O. Kagan, M. Hoopmann, H. Abele, R. Alkier and K. Lüthgens. Ultrasound Obstet Gynecol 2012; 40: 530-535.