Objective To estimate the fetal loss of Down's syndrome fetuses between the time of chorionic villus sampling (10 weeks gestation) and the time of amniocentesis (16 weeks gestation) and term in women aged 35 years and older.
Design The age specific prevalence rates in the first trimester of Down's syndrome were estimated using the Danish cytogenetic register in combination with results from four published studies. These were compared with the reported prevalence at the time of amniocentesis and at birth.
Subjects 5927 singleton pregnancies undergoing chorionic villus sampling (71 cases of Down's syndrome and 5856 unaffected cases). This was combined with published data on a further 231 cases of Down's syndrome and 16620 unaffected cases.
Main outcome measures Age specific prevalences at the time of chorionic villus sampling. Proportion of pregnancies lost between the time of chorionic villus sampling and the time of amniocentesis and term.
Results Thirty-two percent of Down's syndrome pregnancies are lost between the time of chorionic villus sampling (10 weeks) and the time of amniocentesis (16 weeks) and 54% are lost by term.
Conclusions The high fetal loss rates of Down's syndrome between the time of chorionic villus sampling and term introduce problems when evaluating first trimester screening tests with respect to their effective detection rates at term. A recommendation for quoting term risks is made.