HSG and SIS are sometimes discussed as though they are interchangeable. They overlap, but they are designed to answer different questions.
An HSG is often chosen when the main concern is whether the fallopian tubes are open. An SIS gives a closer look at the inside of the uterus.
HSG looks at the uterus and fallopian tubes
During a hysterosalpingogram, a clinician passes a thin tube through the cervix and introduces iodine-based contrast. Fluoroscopy, a moving form of X-ray imaging, shows the shape of the uterine cavity and whether contrast flows through the fallopian tubes.
Cramping ranges from mild to strong and is often most noticeable while the contrast is being introduced. The examination cannot diagnose every cause of infertility, and a result that suggests blockage may sometimes need further assessment.
SIS gives a clearer view of the uterine cavity
During saline infusion sonohysterography, sterile saline is introduced through the cervix while a transvaginal ultrasound is performed. The fluid separates the walls of the uterus so polyps, fibroids, scar tissue, or changes in the shape of the cavity can be seen more clearly.
SIS uses ultrasound rather than ionising radiation. Some versions of the examination can also assess whether fluid passes through the tubes, but a standard SIS is mainly focused on the uterine cavity.
| Feature | HSG | SIS | | :--- | :--- | :--- | | Imaging | Fluoroscopy using X-rays | Ultrasound | | Fluid | Iodine-based contrast | Sterile saline | | Main question | Are the fallopian tubes open? | Is there a change inside the uterine cavity? | | Common sensation | Brief cramping that varies in intensity | Brief cramping or pressure that varies in intensity |
Timing and safety checks
Both examinations are usually arranged after menstrual bleeding has ended and before ovulation, but exact timing varies with cycle length and the centre's protocol. Neither should be performed when pregnancy is possible.
Tell the team about abnormal bleeding, pelvic infection symptoms, a previous reaction to iodine-based contrast, or any chance of pregnancy. Ask your clinician what pain medicine is suitable for you rather than taking it automatically.
After either test, mild cramping or spotting may occur. Seek medical advice for fever, worsening pelvic pain, heavy bleeding, fainting, or foul-smelling discharge.
The bottom line
HSG and SIS answer related but different questions. The test you need depends on whether your clinician is mainly assessing the tubes, the uterine cavity, or both.