Chorionic Gonadotropin, beta Subunit, Human ; blood ; Cost-Benefit Analysis ; Down Syndrome ; blood ; diagnosis ; False Positive Reactions ; Female ; Health Care Costs ; Humans ; Maternal Serum Screening Tests ; economics ; methods ; standards ; Pregnancy ; Pregnancy Trimester, First ; blood ; Pregnancy-Associated Plasma Protein-A ; metabolism ; Prenatal Care ; methods ; Prenatal Diagnosis ; economics ; methods ; standards