The cervical mucus is produced by the cervix and changes consistency throughout your menstrual cycle based on hormones associated with ovulation. Shortly after the menstrual period ends, the cervical mucus is dry and prevents sperm from penetrating the cervix. About a week before ovulation, under the influence of estrogen, cervical mucus becomes abundant, sticky, and thick. Cervical mucus throughout the menstrual cycle based on 28-Day Cycle and in relation to ovulation:
Cervical mucus changes according to ovulation and your menstrual cycle.
There are no significant changes in the cervical mucus which can tell you before a missed period reliably enough whether you are pregnant or not.