What Is Hysterosalpingography (HSG) and How Does It Work?

Hysterosalpingography, usually shortened to HSG, is an X-ray procedure that maps the inside of the uterus and fallopian tubes by filling them with contrast dye. It remains one of the first-line tests in the evaluation of infertility, primarily because it can show whether the tubes are open and whether the uterine cavity has a normal shape. But HSG does more than just take pictures: the procedure itself appears to improve fertility for months afterward, particularly when oil-based contrast is used, a phenomenon that has reshaped how clinicians think about a test traditionally viewed as purely diagnostic.

What Happens During the Procedure

HSG is typically performed in a radiology suite during the first half of the menstrual cycle, after bleeding has stopped but before ovulation. You lie on an X-ray table in a position similar to a pelvic exam. A speculum is placed, and a thin catheter is threaded through the cervix into the uterus. Contrast dye is then slowly injected while a series of X-ray images are taken. The dye fills the uterine cavity, outlines its shape, flows into the fallopian tubes, and, if the tubes are open, spills out their ends into the pelvic cavity. The entire procedure usually takes between 15 and 30 minutes.

Radiation exposure from HSG is modest. A study across multiple imaging centers found that effective doses ranged from roughly 1.2 to 2.5 millisieverts depending on the facility and equipment used, with entrance skin doses between about 15 and 34 milligrays.¹ Fluoroscopy-guided digital techniques reduce skin dose by about a factor of three compared to older radiographic spot-film methods.² More recently, using last-image-capture instead of conventional spot views has shown the potential to cut overall radiation dose by up to 78% without compromising diagnostic quality.³ The estimated cancer risk to the patient from a single HSG is very low, on the order of 24 in a million.

How Well HSG Detects Blocked Tubes

The primary clinical question HSG answers is whether your fallopian tubes are open. In a prospective study comparing HSG with laparoscopy (considered the gold standard), HSG had a sensitivity of 85% and specificity of 96% for detecting a one-sided tubal blockage, and 82% sensitivity with 98% specificity for bilateral blockage.⁴ Those numbers are solid, but they come with an important caveat: false positives are a real issue, especially for blockages near the uterus. In one large series, about 39% of proximal tubal occlusions diagnosed on HSG turned out to be normal at laparoscopy.⁵

The most common culprit behind these false positives is cornual spasm, a transient muscular contraction at the point where the tube meets the uterine wall. On the X-ray images, it mimics a true blockage. Cornual spasm is associated with patient anxiety, a painful procedure, or too-rapid injection of contrast.⁶ When the apparent blockage is on just one side, spasm is an especially likely explanation. Experienced radiologists can sometimes recognize the characteristic rounded shape of the uterine horn that hints at spasm rather than true obstruction. If there is any doubt, the injection is paused, the patient is asked to relax, and a repeat image is taken. When the blockage persists, confirmatory laparoscopy is warranted before making major treatment decisions.

What HSG Shows Inside the Uterus

Beyond the tubes, HSG outlines the uterine cavity itself and can reveal abnormalities like fibroids bulging into the cavity, polyps, adhesions (scar tissue), and congenital uterine anomalies such as a septate or arcuate uterus. However, HSG’s accuracy here is more limited than it is for tubal assessment. One study found that HSG agreed with hysteroscopy (direct camera visualization of the cavity) in only about 57% of fibroid cases, because air bubbles can mimic filling defects and because small or flat lesions are easy to miss.⁷ Another investigation reported an overall HSG accuracy of about 93% for cavity pathology when specificity was high, but sensitivity hovered around 72%, meaning it misses a fair share of real abnormalities.⁸

Congenital uterine anomalies illustrate the limitation well. HSG can detect that the cavity shape is abnormal but often cannot distinguish between a septate uterus and a bicornuate uterus, two conditions that look similar on a flat X-ray image but have different implications and treatments. One series found that five out of six uteri labeled as arcuate on HSG were actually subseptate when examined with combined hysteroscopy and laparoscopy.⁹ In practice, HSG functions as a good screening tool for uterine problems: if the cavity looks normal, it probably is. But when it flags something abnormal, further investigation with hysteroscopy or imaging is usually needed to pin down the exact diagnosis.

The Fertility Boost After HSG

One of the most interesting aspects of HSG is that it is not purely diagnostic. The act of flushing dye through the tubes appears to enhance fertility in the months that follow, and the type of contrast medium used matters substantially. A large randomized trial published in the New England Journal of Medicine found that about 40% of women who received oil-based contrast had an ongoing pregnancy within six months, compared to 29% of those who received water-based contrast.¹⁰ A meta-analysis of randomized trials confirmed this pattern, finding that oil-based contrast was associated with roughly 40-50% higher odds of ongoing pregnancy compared to water-based alternatives.¹¹

The advantage persists over longer follow-up. Five-year data from the same large trial showed that women in the oil-based group had a higher rate of live births (about 75% versus 67%) and conceived naturally more often. Time to ongoing pregnancy was also shorter: roughly 10 months in the oil group compared to nearly 14 months in the water group.¹² A separate meta-analysis pooling multiple studies found increased rates of clinical pregnancy, ongoing pregnancy, and live birth with oil-based contrast, with no difference in miscarriage or ectopic pregnancy rates between the two types.¹³

Why Oil-Based Contrast Seems to Help

The mechanism behind this fertility boost is not fully understood, but several lines of evidence point to immunological changes in the pelvic environment. Oil-based contrast lingers in the peritoneal cavity for months after the procedure, unlike water-based dye, which is absorbed quickly. Laboratory research has shown that oil-based contrast coats peritoneal macrophages, reducing their ability to engulf and stick to cells. This dampening of macrophage activity could create a more hospitable environment for a fertilized egg.¹⁴

More recently, researchers found that oil-based contrast is taken up by dendritic cells in the peritoneal cavity, triggering their maturation and increasing the proportion of regulatory T cells, immune cells that help prevent the body from rejecting an embryo. Samples from women who had undergone HSG with oil-based contrast showed these immune shifts compared to women who had never had the procedure.¹⁵ This immunological remodeling may be especially relevant for women with endometriosis, where an overactive peritoneal immune response can interfere with conception.¹⁶ Beyond the immune effects, the simple mechanical flushing of mucus plugs or debris from the tubes may also play a role, though this would apply equally to both contrast types.

Pain During HSG and What Helps

Ask anyone who has been through an HSG and pain is likely the first thing they mention. Compared to other gynecological office procedures, women rate HSG as more painful and report higher pre-procedure anxiety.¹⁷ Average pain scores vary widely across studies, but many women report scores in the 5-to-7 range on a 10-point scale during dye injection, with pain typically dropping quickly once the procedure ends.

Not all pain-relief strategies work equally well. A randomized trial comparing four approaches found that applying lidocaine spray to the cervix or using a combination of oral painkillers with cervical lidocaine cream reduced pain scores significantly compared to no treatment or oral painkillers alone.¹⁸ Another trial found that combining oral diclofenac with cervical lidocaine cream roughly halved pain scores during dye injection compared to placebo.¹⁹ On the other hand, paracervical block with lidocaine, a technique that works well in other gynecological procedures, showed no benefit for HSG pain in a controlled trial.²⁰

Non-pharmacological approaches have also been studied. Listening to music during the procedure modestly reduced both pain and anxiety scores in a randomized trial.²¹ Virtual reality headsets reduced pain and fear and improved patient satisfaction, though they did not significantly change anxiety levels.²² If you are scheduled for an HSG, asking your clinic about topical cervical anesthesia combined with an oral anti-inflammatory taken beforehand is a reasonable step. What the evidence does not support is taking only ibuprofen or paracetamol by mouth and expecting significant relief from the procedure pain itself.

Infection Risk and When Antibiotics Are Needed

Post-HSG pelvic inflammatory disease (PID) is uncommon, complicating roughly 1 to 2% of procedures overall.²³ But the risk is not distributed evenly. When HSG reveals dilated fallopian tubes, the infection rate jumps dramatically, reaching about 11% in one study, while no infections occurred in women with normal-appearing tubes.²⁴ Women with active chlamydia infection at the time of the procedure face the highest risk: one retrospective review found a 50% rate of post-HSG PID in that group.

Current practice reflects these findings. Women without a history of PID and with no signs of active genital infection generally do not need antibiotics before HSG. But if the procedure reveals dilated tubes, a five-day course of doxycycline is typically prescribed afterward. Women with a known history of PID may receive doxycycline before the procedure as a precaution. A large retrospective cohort study found that antibiotic prophylaxis reduced the risk of post-HSG PID by about two-thirds, with doxycycline showing the greatest protective effect.²⁵ Screening for chlamydia and gonorrhea before the procedure remains a widely recommended step, especially in younger women or those with risk factors.

Contrast Intravasation

Sometimes during HSG, contrast dye enters the blood vessels within the wall of the uterus instead of flowing through the tubes. This is called intravasation, and when it happens, vein-like channels become visible on the X-ray branching away from the uterine outline. Aside from the alarming appearance, intravasation can be clinically significant for two reasons: it tends to be associated with more painful procedures, and severe cases can obscure the tubes on imaging, potentially making the study non-diagnostic.²⁶

Certain factors increase the likelihood of intravasation, including endometriosis, recent fibroid removal, polycystic ovarian disease, and performing the procedure too close to menstruation or ovulation. It was also more common in women with secondary infertility and those with uterine cavity irregularities like adhesions or congenital anomalies.²⁷ When intravasation is spotted, the radiologist typically pauses the injection and allows the contrast to clear before deciding whether to continue. With oil-based contrast, the concern about intravasation is somewhat greater because the oil is not absorbed as quickly as water-soluble dye, but serious systemic complications remain rare.

Thyroid Concerns After Oil-Based HSG

Oil-based contrast agents used in HSG are iodine-rich, and because the oil lingers in the body, the iodine load can be substantial and prolonged. A dedicated study tracking thyroid function after oil-based HSG found that iodine excess was nearly universal, with 98% of women showing elevated urinary iodine levels. In 90% of participants, iodine levels exceeded 1,000 micrograms per liter, and in some women the excess persisted above that threshold for at least three months. About 38% of women developed subclinical hypothyroidism, though the vast majority of cases were mild.²⁸

This matters because thyroid function is closely tied to fertility and early pregnancy outcomes. Subclinical hypothyroidism during early pregnancy has been linked to increased miscarriage risk. Since the whole point of HSG in this population is to help women conceive, the irony of potentially disrupting thyroid function is worth noting. Current recommendations suggest checking thyroid-stimulating hormone levels before and after oil-based HSG, particularly in women with pre-existing thyroid antibodies or borderline thyroid function. Water-based contrast causes far less iodine loading and is not associated with this thyroid effect to the same degree.

Alternatives to Traditional HSG

For women who want tubal assessment without radiation exposure, ultrasound-based methods have become increasingly available. Hysterosalpingo-foam sonography, known as HyFoSy, uses a foam contrast agent injected into the uterus while an ultrasound probe tracks whether the foam passes through the tubes. A multicenter study comparing the two approaches found that HyFoSy was significantly less painful, with a median pain score of 2 out of 10 compared to 5 for HSG, and anxiety levels were also lower.²⁹ HyFoSy avoids radiation entirely and can be performed in a gynecologist’s office rather than a radiology suite, making it more accessible. Its main limitation is that it does not provide the same detailed static images of the uterine cavity that HSG does, and operator skill matters more.

MRI-based hysterosalpingography has also been explored. A meta-analysis of studies comparing MRI with conventional X-ray HSG found that MRI had a pooled sensitivity of 91% and specificity of essentially 100% for detecting tubal occlusion.³⁰ MRI avoids radiation and offers excellent soft-tissue detail, which can help characterize uterine anomalies at the same time. The downsides are cost, limited availability, and the fact that MRI cannot deliver the potential therapeutic flushing effect that comes with conventional HSG.

When HSG Makes the Most Sense Economically

Given that alternatives exist, the question of when HSG is the most cost-effective first step in a fertility workup has been studied explicitly. An economic analysis found that when a couple’s baseline chance of conceiving within three years was relatively low, starting with HSG was the most cost-effective approach. In couples with better fertility prospects, beginning with a simpler and cheaper chlamydia antibody test before deciding whether to proceed to HSG or laparoscopy was the more economical path.³¹ In practice, most fertility clinics still use HSG early in the workup because it gives both tubal and uterine information in one visit, and the procedure itself may improve the odds of conception.

Difficult Access and Cervical Challenges

Not every HSG goes smoothly from a technical standpoint. One of the most common reasons for a failed or incomplete study is difficulty getting the catheter through the cervix. Severe cervical stenosis, an unusually tilted uterus, or scarring from prior procedures can make cannulation difficult or impossible. In some cases, clinicians use cervical softening agents beforehand to make access easier and reduce the risk of perforation. The problem is more common in women who have never been pregnant, whose cervical canal tends to be narrower. When the cervix cannot be navigated, the study may need to be rescheduled after cervical preparation, or an alternative imaging method may be recommended instead.

Even with successful catheter placement, timing matters. Performing HSG too early in the cycle risks doing it during menstruation, which can introduce air bubbles or blood clots that mimic pathology. Performing it too late risks disrupting an early pregnancy. The sweet spot for most clinics is between day six and day twelve of the cycle, after bleeding has fully stopped.