A pregnant women can be a Sotah but only if she is pregnant to her own husbands child.
The Gemorah Sotah 26A says
מְעוּבֶּרֶת חֲבֵירוֹ וּמֵינֶקֶת חֲבֵירוֹ לֹא שׁוֹתוֹת
A woman who was pregnant with the child of another man at the time of her marriage and a woman who was nursing the child of another man at the time of her marriage does not drink the Sotah waters
The Gemorah continues on to say explicitly that she would go through the process if she were pregnant with his child.
In general if a women does a crime that carries the death penalty we do not wait for her to give birth before carrying it out.