Recurrences of free trisomy 21: analysis of data from the National Down Syndrome Cytogenetic Register

OBJECTIVES: To determine the recurrence risk of a free trisomy 21 pregnancy. METHODS: Data from the National Down Syndrome Cytogenetic Register (NDSCR), which contains information on nearly all cases of Down syndrome between 1989 and 2001 in England and Wales were used. Among 11 281 women with a Down syndrome pregnancy who had had at least one previous pregnancy there were 95 women who had had a previous Down syndrome pregnancy. RESULTS: Women who have had a previous Down syndrome pregnancy have a constant absolute excess risk above their maternal age-related risk of having a subsequent affected pregnancy. This absolute excess risk is determined by the age at which the affected pregnancy occurred and is higher for younger than for older women. For example, after a Down syndrome pregnancy at age 20, this excess is 0.62% (95% CI: 0.24 to 1.15%) at early second trimester, and, after one at age 40, it is 0.04% (95% CI: 0.01 to 0.07%). CONCLUSION: More precise risk estimates by single year of maternal age for use in genetic counselling are provided, but they need validation from other studies before they are incorporated in the risk estimation routines used in Down syndrome screening programmes.