Vaginal ultrasound is safe, non-invasive, easy to perform, fast and accurate, and has advantages that other imaging techniques cannot match in the current infertility examination and diagnosis. However, many patients who are having vaginal ultrasound for the first time at fertility clinics are not familiar with this test and may even be apprehensive about it. The following is a detailed description of vaginal ultrasound. Transvaginal ultrasound is a type of intracavitary ultrasound in which a special instrument with a miniature probe at the end of a long head is placed inside the vagina for ultrasound examination. Vaginal ultrasound does involve inserting the probe into the vagina, although it does not enter the uterine cavity. It is only an observational test and does not cause damage to the vagina, much less the uterine cavity. It does not usually cause discomfort or pain to the patient. Advantages of vaginal ultrasound: The frequency of the vaginal probe is higher than that of the probe routinely used in the abdomen, and the resolution is higher than that of the abdominal probe. The probe is placed in the vagina close to the cervix and vaginal vault, allowing a clear sonogram of the pelvic organs, especially for ovarian follicle monitoring, early ectopic pregnancy, early pregnancy, posterior uterus, intrauterine lesions and posterior pelvic masses located in the posterior pelvis. Vaginal ultrasound does not require a full bladder, the pelvic organs are in their natural state and the patient does not have to endure the discomfort of a full bladder. When vaginal ultrasound is performed in obese patients, the image is more clearly displayed because the probe is close to the vault and there is less absorption of the sound beam and no significant attenuation. Transvaginal ultrasound-guided puncture for egg retrieval is a common route in current in vitro fertilization-embryo transfer (IVF) technology. However, vaginal ultrasound has its own limitations: because the focus area of the vaginal probe is within 10 cm, the distant area is not clear. For mid- to late-stage pregnancies, large pelvic masses and uterine fibroids, vaginal ultrasound cannot show the whole picture and abdominal ultrasound is required. Since the vaginal probe needs to be inserted into the vagina for operation, it is not suitable for patients with menstruation, vaginal malformation and vaginitis. Vaginal ultrasound is mainly used in the examination of infertility: 1. To examine the shape, position and size of the uterus, to measure the length, width and thickness of the uterus, etc. It is used to observe the degree of uterine development and the presence of uterine malformations (e.g. no uterus, trace uterus, bicornuate uterus, unicornuate uterus, stump-angle uterus, etc.); 2. To examine the presence of tumors, such as uterine fibroids, ovarian tumors, cystic teratomas, etc.; 3. To monitor ovulation. From 4 to 5 days before the expected ovulation, ultrasound will be done every day or every other day to see the rate of follicle growth and the presence of ovulation. A follicle diameter ≥18mm indicates that the follicle is mature and ovulation is possible at any time. The signs of ovulation are: disappearance or shrinkage of the follicle; a small amount of liquid dark area in the rectal fossa of the uterus; blurring of the follicle margin with sparse light dots inside, and if the light dots are dense and form a light cluster, the corpus luteum. In conclusion, vaginal ultrasound is a non-invasive diagnostic technique that can be performed repeatedly and can make a rapid and more accurate diagnosis, and is an indispensable tool in the diagnostic treatment of fertility.