28 days is normal pregnancy span (god I wish it was for me, I am 6 months preg, got another 3 to go) and she will get a very big swollen belly, and just infront of her bum, she will get a big black area that will grow more and more bigger and black as she gets close.
That black area is the little babies.
And then dont be suprised if she is pregnant again immediatly after, they can hold up to 6 depsosits of sperm in them, and are re impregnanted after birth from those holdings.