Do female dogs become aggressive during pregnancy?

Not really, since "aggressive" connotes "starting trouble." What they become is extremely protective.

While the puppies are helpless, she's going to be very cautious about who comes near them, and may "aggressively defend" her litter to make unwanted intruders (especially other dogs, and particularly "daddy," if he's around) keep their distance. Her protective instinct may extend to people, too, depending on her level of trust, so new pups should always be approached with caution. If the mother starts showing clear signs of agitation, that's her early warning to "back off."

When the puppies start moving around under their own power, she'll start acting more like her usual self again.