People with Down's aren't a national, ethincal, racial, nor religious group so strictly speaking it doesn't count as genocide under international law. But it would meet less formal definitions.
Luckily individual women making a choice to have an abortion cannot be carrying out a campaign of genocide. But when a society pushes women in this direction by failing to provide support or by failing to provide accurate information that society is probably engaging in genocidal actions.
> Article 2 of the convention defines genocide as
> any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:
[...]
> (d) Imposing measures intended to prevent births within the group;
>Generally speaking, genocide does not necessarily mean the immediate destruction of a nation, except when accomplished by mass killings of all members of a nation. It is intended rather to signify a coordinated plan of different actions aiming at the destruction of essential foundations of the life of national groups, with the aim of annihilating the groups themselves. The objectives of such a plan would be the disintegration of the political and social institutions, of culture, language, national feelings, religion, and the economic existence of national groups, and the destruction of the personal security, liberty, health, dignity, and even the lives of the individuals belonging to such groups
Even if the percentage of women electing to have an abortion is high, they're not acting as a singular cohesive body with the collective goal to eliminate all children having Down syndrome.