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A practical solution to combining dating and screening for Down's syndrome

The offer of combined Down's syndrome screening to all pregnant women in England requires accurate estimation of gestational age. The guidance issued by the British Medical Ultrasound Society (BMUS) in 2009 recommends dating by crown rump length (CRL) up to 13 weeks 0 days and by head circumference (HC) after this gestation. The guidance issued by the Fetal Anomaly Screening Programme (FASP) in 2010 states that combined screening for Down's syndrome is the recommended strategy. Combined screening requires the measurement of nuchal translucency, which should be performed when the CRL is between 45.0 and 84.0 mm. Uncertainty exists as to how best to combine the BMUS dating guidance and the FASP Down's syndrome screening guidance in routine practice. The solution proposed by the authors is to date the pregnancy by CRL up to and including 84.0 mm and to date using HC when the CRL is greater than 84.0 mm, irrespective of whether or not Down's syndrome screening is requested. Screening for Down's syndrome should be by combined screening when the CRL is between 45.0 and 84.0 mm. Where the CRL is greater than 84.0 mm, screening for Down's syndrome should be by maternal serum quadruple testing, performed after 14 weeks and two days.