The difference in birth weight between children with Down's syndrome and their siblings was estimated to be in the interval from .18 to .37 kg with 90 percent confidence, the Down's syndrome infants having the smaller mean birth weights. When we adjusted their birth weight, using a multiple linear regression analysis, to take into account several factors known to affect it, the difference in birth weight was only slightly less than the unadjusted difference. The data support the hypothesis that an extra chromosome No. 21 results in a smaller than expected birth weight.
