We conducted a prospective intervention study of screening for fragile X sy
ndrome in the general population. Antenatal and preconceptional screening w
ere carried out in 9459 women aged between 19 and 44 with no known family h
istory of fragile X syndrome. 80% were tested antenatally. 134 carriers wer
e detected (a frequency of 1 in 70); 130 had a premutation (Phl) and 4 had
a full mutation (FM). Prenatal diagnosis was carried out in 108 concurrent
or subsequent pregnancies among carriers involving 111 fetuses. Nine had an
FM, a rate of 1 in 12, two of the affected embryos received the FM directl
y from the mother and in seven it was the result of expansion from a PM. In
all cases with an FM the pregnancy was terminated. In PM carriers there wa
s evidence of a selection against the mutated chromosome with a segregation
ratio of 0.40. Owing to the high rate of premutated chromosomes in our pop
ulation we conclude that screening for fragile X syndrome among women of re
productive age should be more widely available. Copyright (C) 2000 John Wil
ey & Sons, Ltd.