“The authorities perform no autopsy...”
Was she pregnant?
The Kennedy Team pulled a fast one. The law of Massachusetts places the body in the custody of the local Medical Examiner. The Chief Medical Examiner (Dr. Dukakis... Mike's dad) circumvented the practice by calling in the ME from Springfield, 175 miles + a long ferry ride, across the state. Of course, he drove.
In the meantime, The Kennedy Team buffaloed the local funeral home and literally seized the body, which was immediately (and illegally) flown to PA in a private plane, before the worthy medical gentleman and Kennedy stooge from Springfield could arrive on the scene.
The funeral director, experienced in drowning cases, noted that Ms. Kopechne did NOT drown, but had suffocated. Nobody asked him. The imported ME reviewed his notes and made it "Death by Drowning" on the official record, without an autopsy. Later, a MA DA sought exhumation, but was denied permission by a PA Judge, and as I understand it, the Kopechne family was also against further investigation. That may, or may not have, had anything to do with the settlement they accepted from the Kennedys.
BTW, this is absolutely typical Kennedy family behavior around Cape Cod and the Islands. Sometimes it seems there isn't a village or hamlet that doesn't have at least one permanently injured or paralyzed victim of some lesser Kennedy prank, or accident, limping or wheeling about.
Was she pregnant?
Ms. Kopechne was a woman of excellent reputation, so it is unlikely. However, technically speaking, it can never be known one way or the other.