Frequencies of chromosomal abnormalities at amniocentesis: Over 20 years of cytogenetic analyses in one laboratory

Prenatal diagnosis of chromosomal disorders has been performed for more tha n 20 years, mainly for advanced maternal age. Chromosomal abnormality rates derived from second trimester amniocentesis have mainly come from a collec tion of small-scale studies from North America and Western Europe. Accurate risk estimates for chromosomal abnormalities are important tools for the p hysician or obstetrician who would need to make referrals to a prenatal gen etic center. This paper presents amniocentesis rates of clinically signific ant cytogenetic abnormalities for various indications, including advanced m aternal age, previous chromosomal abnormality, parental structural rearrang ement and a family history of aneuploidy as defined in the text. These data come from a Canadian prenatal diagnosis laboratory with more than 20 years experience in second trimester cytogenetic analysis. They show that the ov erall frequency of chromosomal abnormalities for advanced maternal age (gre ater than or equal to 35 years) is 1.79%, In this group, 21% of all abnorma lities are structural rearrangements (including markers) and less than half of all abnormalities are trisomy 21, The advanced maternal age specific ri sk of aneuploidies at second trimester is 1.24%. Recurrence risk for aneupl oidy after a previous one is 1.29%, However, it is much higher (4.84%) for women of greater than or equal to 35 years, When a parent's brother, sister , nephew or niece is affected, the risk of occurrence of aneuploidies (0.24 %) is not elevated. When there is a balanced translocation in one of the pa rents, the overall risk is 10.2% for unbalanced translocations and 37.3% fo r balanced translocations. (C) 1999 Wiley-Liss, Inc.