[StacyRoberts]

That is different because down syndrome is not hereditary. It's caused when chromosomes don't properly separate due their tangling caused by aging of egg cells.

Plus, if it were a hereditary genetic condition, abortion would eliminate those genes from the population reducing the occurrence over time.

[lemmonii]

Would you agree that there are probably some genes, especially on the mothers side which would increase the likelihood of the foetus having down-syndrome?

Or some genes on the mothers side which will increase likelihood of engaging in behaviour that increase the chances of the foetus having down-syndrome? (Smoking etc...)

Because if so, then a huge evolutionary pressure has been lifted from having a down-syndrome child. In the past having such a child would incur immense cost. There are of course difficulties birthing and sustaining an evolutionary dead-end which would reduce the birth-rate of down-syndrome having couples right?

Therefore, genes which increase the likelihood of having down-syndrome will proliferate much more in world which aborts down-syndrome foetuses right?