Chromosomal abnormalities

Chromosomal abnormalities are particularly common in
spontaneous abortions. At least 20% of all conceptions are
estimated to be lost spontaneously, and about half of these are
associated with a chromosomal abnormality, mainly autosomal
trisomy. Cytogenetic studies of gametes have shown that 10% of
spermatozoa and 25% of mature oocytes are chromosomally
abnormal. Between 1 and 3% of all recognised conceptions are
triploid. The extra haploid set is usually due to fertilisation of a
single egg by two separate sperm. Very few triploid pregnancies
continue to term and postnatal survival is not possible unless
there is mosaicism with a normal cell line present as well. All
autosomal monosomies and most autosomal trisomies are also
lethal in early embryonic life. Trisomy 16, for example, is
frequently detected in spontaneous first trimester abortuses,
but never found in liveborn infants.