The internal os is an opening within the cervix, serving as the upper gateway of this structure. It marks the boundary where the cervical canal meets the main body of the uterus. This opening acts as a portal between the uterine cavity and the lower reproductive tract. Its function is important for various reproductive processes.
Anatomical Context and Primary Role
The cervix is the lower, cylindrical portion of the uterus, connecting the main uterine cavity and the vagina. It contains a narrow central canal, known as the endocervical canal. The internal os forms the opening of this canal into the uterus, while the external os leads into the vagina.
In a non-pregnant state, the internal os functions as a protective barrier. It guards the uterus from external pathogens. The internal os also regulates the flow of menstrual blood out of the uterus. The cervix is composed primarily of dense connective tissue, including collagen and elastic fibers, with less muscle tissue than the rest of the uterus, making it relatively rigid.
Its Role in Pregnancy
The internal os plays a significant role in maintaining pregnancy. Throughout gestation, it remains tightly closed and firm, securing the fetus within the uterus. This closure prevents the premature expulsion of the fetus as intrauterine pressure increases.
The closed internal os also acts as a protective shield against ascending infections. During pregnancy, a thick mucus plug forms within the endocervical canal, sealing off the uterine environment. This barrier function is essential for maintaining sterile conditions for fetal development.
Changes During Labor and Delivery
As labor approaches, the internal os undergoes a transformation. This process begins with “effacement,” where the cervix, including the internal os, thins and shortens. This thinning, often described as ripening, prepares the cervix for dilation.
Following effacement, “dilation” occurs, where the internal os gradually opens. This widening allows the baby to pass from the uterine cavity into the birth canal. These changes in the internal os and cervix are driven by uterine contractions and hormonal shifts, facilitating birth.
Common Conditions and Implications
Several medical conditions can arise when the internal os does not function as expected. One such condition is cervical insufficiency, sometimes referred to as an incompetent cervix. In this scenario, the internal os opens prematurely during pregnancy, often without contractions or pain, potentially leading to preterm birth or miscarriage. This premature opening can expose the amniotic sac and fetus to the vaginal environment.
Another condition is cervical stenosis, where the internal os is abnormally narrowed or even completely closed. This narrowing can impede the natural flow of menstrual blood, potentially causing painful periods or difficulty with fertility by obstructing sperm passage. During labor, cervical stenosis can hinder the normal dilation process, potentially prolonging labor or necessitating interventions. Diagnosis often involves ultrasound imaging, and management for conditions like cervical insufficiency may include a cerclage, a surgical procedure to reinforce the cervix.