Fetal nuchal translucency thickness at the 11(+0) - 13(+6) weeks scan has been combined with maternal age to provide an effective method of screening for trisomy 21; for an invasive testing rate of 5%, about 75% of trisomic pregnancies can be identified. When maternal serum free B-human chorionic gonadotropin and pregnancy-associated plasma protein-A at 11(+0) - 13(+6) weeks are also taken into account, the detection rate of chromosomal defects is about 85-90%. In 60-70% of fetuses with trisomy 21 the nasal bone is not visible at the 11(+0) - 13(+6) weeks scan and examination of the nasal bone can increase the detection rate of screening by the first trimester scan and serum biochemistry to more than 95%.