Anyone know if a congenital anomaly in the cervix is dangerous for childbirth?

Usually not: There are different types of anomalies, but the most common ones are duplication of the reproductive tract. These anomalies increase the frequency of breech babies at term and sometimes are a worry for preterm labor, but overall are usually safe.

Depends: The term congenital anomaly is a broad one. Anomalies of the female reproductive system vary from a complete absence of the uterus and cervix, to mild changes, like a a heart shaped uterus with normal cervix. The effect on reproductive ability depends on the defect involved.