SIS (Saline Infusion Sonography)

SIS stands for Saline Infusion Sonography — a procedure where sterile saline is gently injected into the uterine cavity through a thin catheter during a transvaginal ultrasound scan. Also called saline sonogram or sonohysterography. It outlines the inner walls of the uterus with fluid, making it possible to identify polyps, submucosal fibroids, adhesions, and uterine septa that would otherwise be missed on a standard ultrasound scan.

A regular pelvic TVS can see the uterine walls but cannot reliably detect small intracavitary lesions (polyps, thin adhesions, small submucosal fibroids) because the endometrial surfaces are compressed together. SIS distends the cavity with saline, separating the walls and revealing lesions floating in the fluid — increasing sensitivity for polyps from ~65% (standard TVS) to ~95% (SIS). It adds approximately 5–10 minutes to a standard TVS appointment.

SIS has sensitivity ~95% for endometrial polyps — comparable to hysteroscopy (sensitivity ~98%). SIS has the advantage of being outpatient, requiring no anaesthesia, lower cost, and also providing information on ovaries and AFC. Hysteroscopy allows simultaneous visualisation and removal of the polyp. The standard clinical pathway: SIS to diagnose → hysteroscopy to treat. SIS is the preferred first-line diagnostic tool before IVF; hysteroscopy is reserved for when treatment is needed.

SIS is performed on Day 5–10 of the menstrual cycle — the post-menstrual, pre-ovulatory window. This timing ensures: (1) the endometrium is thin (easier to see lesions against saline background); (2) no risk of disrupting an early pregnancy; (3) pre-ovulatory so no active corpus luteum. Avoid during menstruation (blood obscures view) and after ovulation (endometrium thickens, reduces sensitivity). Most fertility centres perform SIS on the same Day 2–4 visit as the baseline scan + blood draw.

Antibiotic prophylaxis is not universally recommended for SIS in low-risk women. However, some centres prescribe a single dose of doxycycline (100mg) before the procedure for women with a history of pelvic inflammatory disease (PID), previous pelvic surgery, or known tubal pathology — as introducing saline could theoretically activate dormant pelvic infection. There is no high-quality evidence for routine prophylaxis in all patients. Post-procedure: mild spotting and cramping for 24–48 hours is normal; any fever or severe pain should prompt urgent review.