Trisomies 13 and 18: population prevalences, characteristics, and prenatal diagnosis, metropolitan Atlanta, 1994-2003

In recent years, prenatal diagnosis and elective pregnancy termination have affected the reported birth prevalence of trisomies 13 and 18. We examined the prevalence and characteristics of these conditions using 1994-2003 data from a population-based surveillance system, the Metropolitan Atlanta Congenital Defects Program. Including fetal deaths and elective terminations increased the number of affected pregnancies by 58.7% for trisomy 13 and 72.2% for trisomy 18. Prenatal cytogenetic testing was reported in 70.8% of trisomy 13 cases and 76.1% of trisomy 18 cases. Among those with prenatal cytogenetic tests, 60.8% of trisomy 13 and 59.7% of trisomy 18 cases were electively terminated. Compared with non-Hispanic whites, non-Hispanic black race was associated with a decreased frequency of prenatal cytogenetic testing for both trisomy 13 and trisomy 18 (OR 0.24, 95% CI: 0.08-0.78 and OR 0.32, 95% CI: 0.14-0.69, respectively). The reported rates of prenatal cytogenetic testing remained stable throughout the period. As expected, maternal age > or =35 years was a risk factor for both conditions. However, while 67.1% (n = 55) of the trisomy 18 case mothers were > or =35 years, only 46.9% (n = 15) of the trisomy 13 case mothers were > or =35 years. Among live-born infants, the sex ratio among trisomy 18 infants showed an increased proportion of females: 60.4% female versus 39.6% male. However, the proportion was 48.3% female and 51.7% male among fetuses that were electively terminated in the second trimester. Inclusion of pregnancies that are prenatally diagnosed is critical for accurate surveillance and population-based analyses of these conditions.