Response to: Wald, N.J., Hackshaw, A.K. (1997). Combining ultrasound and biochemistry in first-trimester screening for Down's syndrome, Prenat. Diagn., 17, 821-829 [letter]MeSH TermsChorionic Gonadotropin, beta Subunit, Human/bloodDown Syndrome/diagnosis*FemaleHumansNeck/ultrasonographyPregnancyPregnancy Trimester, FirstPregnancy-Associated Plasma Protein-A/analysisPrenatal Diagnosis/methods*