You can not get pregnant by swallowing someone else’s saliva.
You could drink saliva by the gallon and while it might make you nauseated in a way that, theoretically, could simulate morning sickness, it would not make you pregnant.
For the record: the simulation of morning sickness would be more accurate if you drank the gallons of saliva every day. Most people get morning sick on a regular basis, not just once.
That said, if you had access to gallons of saliva to drink as part of your regular morning routine, I would be concerned that you were part of a vast criminal conspiracy that enticed men to drool into cups by showing them pictures of scantily clad women.
Explanation for why you can’t get pregnant by drinking saliva:
There are no sperm in saliva. I repeat. There are no sperm in saliva. Since you can not get pregnant without sperm, there is no way for saliva to make you pregnant.
In addition, even if there were sperm in the saliva you were swallowing, there would be no way for it to get from your stomach to your uterus.