The [1] is the narrow neck of the uterus that keeps the uterus closed and the fetus within during pregnancy.

The structure you are referring to is the cervix. The cervix is the lower part of the uterus that extends into the vagina. It acts as a passageway for menstrual fluid to exit the body and serves as the birth canal during childbirth. The cervix contains mucous glands that produce cervical mucus, which changes in consistency during the menstrual cycle and influences fertility. During pregnancy, the cervix remains tightly closed to protect the developing fetus from infections and to maintain the pregnancy until the time of labor. As labor approaches, the cervix undergoes changes, softens, and begins to dilate, allowing the baby to pass through during delivery.