Observed versus expected rates of unbalanced fetal karyotype at second trimester amniocentesis when one parent carries a balanced translocation.

Our purpose is to evaluate a model proposed by Stene et al. for estimating the risk of unbalanced progeny at the time of second trimester amniocentesis for 114 pregnancies of 71 couples, who are carriers of a reciprocal translocation. The overall estimated risk was 18%, 1.26 times higher than the observed risk (14%). There were no unbalanced fetuses in the 19 pregnancies of the no-risk group. In the low-risk group, the observed risk was highly correlated with the estimated risk. However, the estimated risks in the medium- and high-risk groups were much higher than the observed risks (2- and 1.5-fold). In the group ascertained through spontaneous abortions, the average estimated risk was 1.9 times higher than the observed risk. Although the model is useful in estimating individual empirical risks, it may be necessary to consider the type of ascertainments and add some modifications to calculate the risks in the second trimester.