Book about Teen Pregnancy?

I've been trying to remember the title of this book for years, so any help will be greatly appreciated. It's about two thirteen year old kids who are experimenting with their sexuality. The boys mother encourages them to have sex since the girl hasn't started her period yet, and she demonstrates the mechanics of intercourse using a taco shell. The girl ends up getting pregnant and they consider abortion, I think, but ultimately keep the baby. It was the first in a trilogy, I'm pretty sure. The second one was set 8 years later. I can't remember anything else helpful, but hopefully someone else has read this book, because I'm beginning to think I imagined its existence.