Observed frequency of fetal trisomy between 16 and 24 gestational weeks in pregnant women older than 34 years at delivery

ABSTRACT

PURPOSE: Increased maternal age is a major risk factor for chromosomal abnormalities. The maternal age-specific risk of fetal trisomy was theoretically calculated. We investigated the actual frequency of fetal trisomy between 16 and 24 gestational weeks in pregnant women over the age of 34 at delivery. MATERIALS AND METHODS: We retrospectively, over a four-year period, reviewed the medical records of women with singleton pregnancies that started their antenatal care before the 10th week of pregnancy. Pregnant women aged 34 to 45 years at the time of delivery were enrolled and divided into groups of one-year intervals. We investigated the frequency of Down syndrome and all trisomies as a function of the maternal age and compared with the theoretical maternal-age-specific risk. RESULTS: Of the 5,858 pregnant women enrolled in the study, the rate of trisomy 21 was 0.29% (17 cases). The observed frequencies of trisomy 21 in women with maternal ages of 35 years and 40 years were 11,116 and 1141, respectively. The rate of all trisomies was 0.39% (23 cases). The observed frequencies of all trisomies in women with maternal ages of 35 years and 40 years were 1372 and 156, respectively. CONCLUSION: The frequencies of Down syndrome and all trisomies were proportional to the maternal age. However, the observed frequencies of Down syndrome and all trisomies between the 16 and 24 gestational weeks were lower than the theoretical rates.