The internal os is part of the cervix. The cervix forms the neck of the uterus, which is part of the female reproductive system. Running through the cervix is an opening called the os. The end that opens into the vagina is called the external os, and the end that opens into the uterus is called the internal os. The cervix allows for the passage of menstrual blood and semen, inhibits the passage of harmful organisms into the uterine cavity, and forms the birth canal, which a fetus passes through to the outside world.

The internal portion of the cervix and uterine cavity are covered with stratified squamous epithelium. This layer secretes mucus, which serves several purposes. Around the time of ovulation, the mucus thins out, allowing sperm to move easily through the vagina into the uterus while providing protection against the harsh, acidic vaginal environment. At all other times, the cervical mucus is thick, preventing the movement of sperm and harmful organisms.

Under the epithelial lining, the cervix is muscular. The smooth cervical muscles allow it to dilate and contract. The internal os is about 0.27 inches (about 7 millimeters) wide in a non-pregnant woman of childbearing age. The most extreme dilation occurs during childbirth, when the opening reaches 3.93 inches (about 10 centimeters).

The os is usually firmly closed in non-pregnant females who have never given birth, and slightly open in women who have given birth. The closure helps keep bacteria from the vagina from entering the uterus. During menstruation the os opens slightly to allow the uterine lining to pass. It also opens during ovulation, to allow sperm to pass through, and it dilates during childbirth to allow the baby to pass through.

During pregnancy, the internal os sometimes begins to dilate before the fetus has reached term. This is called funneling, and it means the cervix is incompetent. An incompetent cervix cannot support the fetus as it gets bigger, and most of these pregnancies are lost before the fetus is able to survive outside of the womb. Funneling can be diagnosed using an internal ultrasound. Bed rest may be used in an attempt to bring the pregnancy to term, or the external portion of the os may be stitched.

Placenta previa is a complication of pregnancy in which the placenta partially or completely covers the internal os. If labor occurs, the placenta can tear away from the uterus prematurely, causing profuse bleeding. Most cases are diagnosed early, and a cesarean delivery scheduled. Ideally, the placenta should lie at least 0.79 inches (about 2 centimeters) away from the internal os.