During intercourse, way before ejaculation occurs, a fluid escapes from the male sex organ. This is in layman terms known as Pre-cum.
Research has proven that Pre-cum is capable of fertilizing ova. If implantation of the ova further takes place then you are pregnant!
Unprotected sex is basically calling pregnancy unto thyself because he need not ejaculate to get you pregnant.
Now you know, try to be more careful next time.