18 weeks pregnant. dr. said he felt pressure on my cervix during pelvic exam. will this cause me to dialate or go in preterm labor?

Not really: Risk factors for cervical incompetence and/or pre-term labor (e.g. History of pre-term labor, shortened cervix sometimes, cervical surgery, etc...) should be evaluated.

Hard to say: It might help to get an ultrasound to measure the length of your cervix. That's a more objective way to test whether you are at risk for preterm birth. If your cervix is short, vaginal Progesterone may help.