A small cervical opening can be caused by physiological reasons, cervical adhesions, infantile uterus and so on. 1. Physiological reasons: for women who are thin and small in body size, the reproductive organs also match with the body and develop relatively small, the uterus is small, so the cervical opening is correspondingly small, which is a normal physiological phenomenon, and the secondary sexual characteristics and ovarian function are normal, and it does not affect the birth of a child. 2. Cervical adhesion: due to cervical conization surgery or multiple abortions, cervical adhesion will be caused, making the cervical opening smaller and tighter, which may cause secondary infertility, menstrual blood can not flow out of the uterus and other complications, at this time, need to carry out cervical dilatation, can be combined with the history of the disease, physical examination, ultrasonography and so on to assist in the diagnosis. 3. Infantile uterus: this is a kind of uterine dysplasia, the uterus has normal uterine cavity and endothelium, the ovaries tend to develop normally, the examination shows that the uterus is small in size, the cervix is relatively long and the mouth of the uterus is relatively small, which can be manifested as scanty menstruation or delay in the menarche and may be accompanied by menstrual cramps, which may be diagnosed by combining with the physical examination and ultrasonography. When the cervical opening is relatively small, the cause can be identified under the guidance of the doctor and treated promptly.