degree of MR in parents is not known from your message. If both
parents have MR, the probability of a child having it is statistically
high (according to some publications, the chances are about 50%).
Mental Retardation is not an inheritable disorder, that is if
the mother has MR, it does not mean that her child will have MR.
However, the chances are higher than in the general population.
First, the genetic basis is weak, second, prenatal and postnatal
care may be poor due to the mother's cognitive abilities. Add
to this the possibility of the father having MR. We are talking
about a higher than normal probability, but it is not a sure thing
at all. You wrote: "Olga had significant delays when adopted,
but has all but caught up except for expressive language (approximately
10 month delay) and some fine motor deficiency". I can tell
you only that language and motor skills are the best indicators
of the child's development at the age of three. However, it may
be too early for any clinical prognosis.