Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

improvements in the quality of antenatal carg and in the treatment
of certâ1n deformîtles after birth have produced an absolute increase
ln the population of affected children.
1.3 tNctpENcE oF B!¡I!_!!!!!I!
There is an extensivc Iiterature on the'incidence of bi ¡-ih defects
but the statistics are subject to mâny sources of error, such as:
a) the unknovln rate of prenatal loss
b) incomplete d i agnos is
c) under-reporting of defects
d) vâriations în recordíng methods
è) the preponderãnce of dâta from hospital series,
ln addition, the figures are greatly êltered by the inclusîon or
exclusìon of stillbirths (Keonedy, l!6/; Persaud , 1g7il. ln an international
survey of two huridred and thirty-eight reports,covering twenty
million b¡rths, congenital defects occurred ¡n one to five per cent of
líve births according to the cr¡teria used (Kennedy, 1!6/).
one of the most meticulous investigations of the incidence of
bîrth defects by the World Heal th Organization has achieved a high degree
of comparabil'ity between twenty-four centres in sixteen countries, by
uniform recording methods and standardization of frequencies for,maternal
age (Stevenson et al ., 1966).
t.4 coNer¡!!rA!_lALroRr{nloHs or rú¡ cr¡lrRnl NeRvous sysreil
Abnormal development of the central nervous system may produce a
wide range of malformations of the braln and spinal cord. A simple
class¡fication of these defects, however, ís difficult to achieve because