Carrie begins and ends with blood. And there's a quite a lot of it throughout the middle, too. So where shall we begin with this symbol?
Well, blood is red, obviously—and red is often a symbol of a few human emotions. Anger and lust are the most common of these. (What does that say about Santa Claus' outfit, we wonder?) But it's also just generally a bad omen.
Foreboding. You know, because if someone's bleeding, that's usually not a good thing.
Carrie's mom believes blood is "always at the root of it, and only blood could expiate it" (2.233). It, here, is both power and evil; the two things are one and the same for Margaret White. What's scary is that Margaret is right in this instance.
Blood marks the beginning of Carrie's puberty and her psychic powers. And, just as Carrie is starting to discover that she has other, more normal powers—she's charming and her date seems to actually like her at the prom—she gets pig's blood dumped on her.
At this point, Carrie is kind of sick of having blood on her and being laughed at simultaneously. So she unleashes her psychic powers on her classmates and teachers. And then on the whole town. In that sense, the pig's blood serves as a beginning and an end to the novel.
Carrie gets her period after voting for herself for Prom Queen—and after Tommy says, "To the devil with false modesty" (2.310). Is that a sign that Carrie actually has committed a sin?
Margaret bleeds too, almost at the same time as her daughter does. But that's because she cuts herself when she's planning to kill Carrie. Let's reword that: when Margaret sharpens a knife to kill her own daughter.
But does she take her blood as a sign that maybe she shouldn't kill Carrie? Nope.
Both women ignore this bad-omen blood. Carrie votes for herself again, and wins Prom Queen, setting the events of the whole night into motion. And her mother still tries to kill her. Didn't Carrie and Margaret they learn anything about seeing signs from Ace of Base?