What made you want to look up low-pressure? Please tell us where you read or heard it (including the quote, if possible).
While spaying is traditionally recommended for kitties between 4 and 6 months of age, spaying as young as 8 weeks old is considered safe, according to Cat Channel. Many animal shelters actually spay kitties prior to adopting them out at this age.
When her kittens are weaned and her milk dries up she can get pregnant again. When the kittens are about 6-8 weeks old you need to get her spayed before letting her out. Don't rehome the kittens till 8-12 weeks old so mom can teach them survival and social skills they'll need to become well adjusted cats.