Authors' Reply

Spencer K Coombes EJ Mallard AS Milford Ward A Free beta human choriogonadotropin in Down's syndrome screening: a multicentre study of its role compared with other biochemical markers. Ann Clin Biochem 1992; 29: 506–18

Spencer K Macri JN Aitken DA Connor JM Free beta hCG as first trimester marker for fetal trisomy. Lancet 1992; 339: 1480

Spencer K Macri JN Anderson RW Aitken DA Berry E Crossley JA Dual analyte immunoassay in neural tube defect and Down's syndrome screening: results of a multicentre clinical trial. Ann Clin Biochem 1993; 30: 394–401

Macri JN Spencer K Aitken DA Ganer K Buchanan PD Muller F First trimester free beta Down syndrome screening. Prenat Diagn 1993; 13 (in press)

Aitken DA McCaw G Crossley JA Berry E Connor JH Spencer K First trimester biochemical screening for fetal chromosome abnormalities and neural tube defects. Prenat Diagn 1993; 13 (in press)

Cuckle HS Lilford R Antenatal screening for Down's syndrome. BMJ 1992; 305: 1017

Spencer K Screening for Down's syndrome. The role of intact hCG and free subunit measurement. Scand J Clin Lab Invest 1993; 53 (suppl 206) 79–96

Crossley JA Aitken DA Connor JM Second trimester inconjugated oestriol levels in maternal serum from chromosomally abnormal pregnancies using an optimised assay. Prenat Diagn 1993; 13: 271–80

Kodak (ADL) Trial Results Manual—A Retrospective Study, MAN.4031. London: Kodak, 1991

Cuckle H Measuring conjugated estriol in maternal serum to screen for fetal Down syndrome (editorial). Clin Chem 1992; 38: 1687–9

Reynolds T John R Comparison of assay kits for unconjugated estriol shows that expressing results as multiple of the median causes unacceptable variation in calculated risk factors for Down syndrome. Clin Chem 1992; 38: 1888–93

Reynolds T John R Spencer K The utility of unconjugated estriol in Down syndrome screening is not proven. Clin Chem 1993; 39 (in press)

Reynolds T Practical problems in Down syndrome screening: what should we do about gestation dating? What is the effect of assay imprecision. Commun Lab Med 1992; 1: 31–8

Seth J Ellis AR Sturgeon CM UKNEQAS for Peptide Hormones and Related Substances: Annual Review 1992. Edinburgh: UKNEQAS, 1993

Wald N Cuckle H Densem J Kennard A Smith D Maternal serum screening for Down's syndrome: the effect of routine ultrasound scan determination of gestational age and adjustment for maternal weight. Br J Obstet Gynaecol 1992; 99: 144–9

Wald N Kennard A Densem J Cuckle HS Chard T Butler L Antenatal maternal serum screening for Down's syndrome: results of a demonstration project. BMJ 1992; 305: 391–4

Haddow J Palomaki G Knight G Williams J Pulkkinen A Canick JA Prenatal screening for Down's syndrome with use of maternal serum markers. N Engl J Med 1992; 327: 588–93

Wald N Cuckle H Densem J Nanchatral K Royston P Chard T Maternal serum screening for Down syndrome in early pregnancy. BMJ 1988; 297: 883–7