One cannot get pregnant without sperms being involved. However, there are cases of premature orgasms that can lead to fertilization in the ovaries that result to pregnancies. You should get tested to be sure on this.
Even if your partner withdrew and you think sperm was not released or if he did not release sperm because he did not reach that point, there still could have been sperm released. As you have sex, he is releasing small amounts throughout the whole experience.