How can I use parentheses in google scholar?
As you correctly recognized, Google Scholar doesn't understand parentheses. I don't think there is a solution other than making two separate searches and considering the results of both. If you wish to get a literature overview, that should be sufficient.

If you wish to count the number of hits, you may want to make a third search for intitle:preeclampsia  AND intitle:toxemia AND intext:pregnan* and use the inclusion-exclusion formula: |A or B| = |A| + |B| - |A and B|. (Warning: the number of results is just approximated in Google searches, and this method may return numbers that are way off).

If you wish to do something else, just tell us in the question next time. :)