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The number of babies born each year affected with Down syndrome is the result of several factors, including the number of fetuses conceived that carry the third copy of chromosome 21 are more likely to conceive Down syndrome-affected foetus, and the childbearing population in the percentage of pregnant mothers who choose to test for Down syndrome, and the percentage of women who learn they are carrying a fetus affected with Down syndrome who choose to terminate.

Foetus affected with Down syndrome are more likely to miscarry than normal foetus, but this hasn't changed over time. Interestingly, the number of babies born every year affected with Down syndrome has increased slightly Studies of Down's syndrome covering a period of 9 years revealed an incidence of 1 in 865 livebirths in a Nigerian hospital. Cytogenetic analysis in 386 patients showed 369 (95.5%) cases to be the result of regular trisomy 21, and translocation trisomy 21 was found in nine (2.5%) patients. Six (1.5%) patients were mosaics and the remaining two (0.5%) cases were classified as miscellaneous.

A high occurence of cases among young mothers was recorded, but a search for environmental factors contributory to non-disjunction in this relatively young age group was unrevealing. The study has shown that Down's syndrome occurs as commonly in Nigeria as in other races. Epidemiological studies like this are necessary to heighten the awareness of health planners in communities that have for a long time considered haemoglobinopathies to be the major genetic disorder, in order to prepare the ground for preventive measures.