What month does cervix start to opening at in pregnancy?

The cervix: begins to "efface" (thin out at the outlet) during the third trimester...it does NOT dilate until shortly before birth Hope this helps Dr Z

No specific month: Except for the 9th month of pregnancy, there is no predictable month that the cervix starts to open. However, if the cervix is dilated more than 2 cms, especially if it's also thinning out and the fetus's head can be easily felt on internal exam, it suggests that labor is imminent. If you are less than 37 weeks along, then your OB should take appropriate actions to forestall premature labor.