Each pregnancy carries a small risk of foetal chromosomal abnormalities. The incidence of trisomy 21, which causes Down's syndrome, is less than 1/1000 if the mother is below 30 years of age, and more than 1/100 if the mother is over 40 years of age. Older pregnant women (35-40 years of age) should be offered chorionic villus sampling or amniocentesis to screen for chromosomal abnormalities. Down's syndrome and other chromosomal abnormalities can be screened in women in any age based on age, serum markers or minor abnormalities of the foetus discovered by ultrasonography. One possible strategy for screening of chromosomal abnormalities is to use combined screening in early pregnancy (serum markers at 8-11 weeks of gestation and measurement of nuchal translucency as a part of routine ultrasound examination at week 10-12) or alternatively with maternal serum triple screening at week 14 or 15. Remember that screening is voluntary and a pregnant woman can choose whether she wants foetal diagnostic studies after she has been given information on the reliability, risks, timing of the studies and the reporting methods.