Difference between non pregnant and pregnant cervix?

Shortening: A cervix becomes shorter late in pregnancy and eventually the os (opening) starts to open in the final weeks leading up to labor. Early in pregnancy though there is no difference, and it's not possible to tell if a woman is pregnant from the appearance of the cervix (but you can tell if a woman has had a vaginal delivery before because the os will be larger)

"Plugging" in answer: The major differences between the non pregnant and pregnant cervix is that the pregnant cervix is thicker, and secretes a mucous 'plug" to block bacteria from ascending into the uterus.