Basic Trope: A woman gets pregnant on purpose, in an attempt to keep her boyfriend or husband with her.
Straight: Alice tells Bob that she's on the Pill (she isn't), and that they don't need to use a condom. She gets pregnant and tells him that the Pill apparently didn't work, when in reality she was never on it in the first place.
Alice has done this to two other guys before Bob; obviously, it didn't work.
Alice performs oral sex on Bob, and inseminates herself with the collected semen behind Bob's back.
"Proposing" marriage this way is considered the norm.
Bob gets out of this relationship and moves on, and finds a new girlfriend who treats him with love and respect. He may pay child support if the child is indeed his (assuming Alice doesn't abort it), but otherwise never has anything to do with Alice again.
Played For Laughs: Alice has been taking baby aspirin instead of birth-control pills. Not because anyone's been tampering with them, but because she's Too Dumb to Live.
Played For Drama: Bob doesn't want to stay with Alice, and commits suicide (or flees the country) to avoid paying child support. Meanwhile, Alice's parents kick her out onto the street.
Please don't leave me to go back to The Baby Trap; I'm pregnant with your child!