Chlamydia and gonorrhea are the two STIs most likely to cause infertility, but they’re not the only ones. Mycoplasma genitalium, syphilis, and trichomoniasis can also damage reproductive function in both men and women. The common thread is that untreated infections climb deeper into the reproductive tract, triggering inflammation and scarring that can permanently block the pathways eggs and sperm need to travel.
Chlamydia: The Leading Cause
Chlamydia is the single biggest STI-related threat to fertility, largely because it so often goes unnoticed. It can infect the fallopian tubes without producing any symptoms at all. Left untreated, about 10 to 15 percent of women with chlamydia will develop pelvic inflammatory disease (PID), an infection of the uterus, fallopian tubes, and surrounding tissue.
The damage happens through a cascade of inflammation. When chlamydia reaches the fallopian tubes, the body mounts an immune response that, ironically, does most of the harm. Inflammatory cells flood the area, breaking down tissue and triggering fibrosis, a process where scar tissue replaces healthy tissue. Over time, this scarring can physically block the fallopian tubes or destroy the tiny hair-like structures (cilia) that help move eggs toward the uterus. The damage spreads beyond just the infected cells, affecting surrounding tissue through chemical signaling between neighboring cells.
Each episode of PID roughly doubles the risk of permanent tubal damage, whether the infection causes obvious symptoms or not. Repeated or chronic infections make things progressively worse. In one large study following women for seven years, about 19 percent were categorized as infertile. Women who had recurrent PID were 1.8 times more likely to be infertile than those who had a single episode.
Chlamydia also harms male fertility. It causes lower sperm concentration, reduced motility, and abnormal sperm shape. The bacteria can directly damage sperm DNA, though this damage appears to improve once the infection is treated. In more serious cases, chlamydia causes epididymitis, an infection of the coiled tube behind each testicle where sperm mature. Chronic inflammation there can create scar tissue that blocks sperm from reaching semen. If both sides are affected, the result is complete obstructive infertility.
Gonorrhea: Similar Path, Faster Damage
Gonorrhea causes infertility through essentially the same mechanism as chlamydia: ascending infection, PID, and tubal scarring. The CDC identifies both chlamydia and gonorrhea as the most important preventable causes of PID and infertility. Gonorrhea tends to produce more noticeable symptoms than chlamydia, but not always, and delayed treatment gives the infection time to cause lasting harm.
In men, untreated gonorrhea is particularly destructive. Acute epididymitis from gonorrhea often results in poor semen quality with decreased sperm count and motility. If the infection isn’t treated promptly, the scarring and blockage of the epididymal canal that follows is usually permanent, even after the infection itself is cured. Bilateral blockage (both sides) results in azoospermia, meaning no sperm are present in semen at all.
Mycoplasma Genitalium: A Newer Concern
Mycoplasma genitalium is a less well-known bacterium that has drawn increasing attention from researchers. It causes 20 to 30 percent of male urethritis cases and is associated with a two-fold increased risk for cervicitis, PID, and preterm delivery in women. Studies have linked it to tubal-factor infertility even after accounting for prior chlamydia infection, suggesting it independently contributes to reproductive damage. Research has also found a two-fold increased risk for ectopic pregnancy among women with the infection in their fallopian tube tissue.
Like chlamydia, mycoplasma genitalium causes scarring through inflammatory immune responses in the reproductive tract. In men, it has been associated with decreased sperm concentration, and its negative effects on sperm quality (concentration, motility, morphology, and DNA integrity) may be even more pronounced than those of chlamydia.
Syphilis and Trichomoniasis
Syphilis doesn’t have a direct toxic effect on fertility in the way chlamydia does, but its complications can still cause infertility. Syphilitic epididymitis can lead to obstruction of the reproductive ducts in men, resulting in the same kind of scarring and blockage seen with gonorrhea. The immune responses triggered by syphilis can damage epithelial tissue and cause fibrosis in the reproductive tract.
Trichomoniasis, caused by a parasite rather than bacteria, has been linked to decreased sperm motility, reduced viability, and abnormal sperm shape. While it’s not typically grouped with the major infertility-causing STIs, it can contribute to reduced fertility, especially when combined with other infections or left untreated for extended periods.
How Ectopic Pregnancy Fits In
STI-related tubal damage doesn’t just prevent pregnancy. It can also cause life-threatening ectopic pregnancies, where a fertilized egg implants in a damaged fallopian tube instead of the uterus. Women with a history of chlamydia are more than four times as likely to have an ectopic pregnancy compared to women without prior infection. This happens because partial scarring can narrow the tube enough to trap a fertilized egg without fully blocking it.
Why Screening Matters So Much
The most dangerous aspect of STI-related infertility is that it usually develops silently. Chlamydia can cause “silent” infections in the upper genital tract, permanently damaging the fallopian tubes, uterus, and surrounding tissues without ever producing pain, discharge, or any other warning sign. By the time someone discovers infertility, the scarring may already be extensive.
The CDC recommends annual chlamydia and gonorrhea screening for all sexually active women under 25, and for women 25 and older who have risk factors like a new partner, multiple partners, or a partner with an STI. Pregnant women under 25 should be screened at their first prenatal visit and again in the third trimester. Men who have sex with men should be screened at least annually, or every three to six months if at higher risk.
The logic behind these guidelines is simple: catching and treating these infections early, before they ascend into the upper reproductive tract, prevents the inflammatory damage that leads to infertility. Once scar tissue has formed in the fallopian tubes or epididymis, antibiotics can clear the infection but cannot reverse the structural damage.
Can STI-Related Infertility Be Treated?
Once tubal damage has occurred, the options narrow considerably. Surgical repair of blocked fallopian tubes can restore fertility in some cases, with one study reporting an overall pregnancy rate of about 55 percent after laparoscopic repair, though this was specifically for women who had undergone tubal sterilization rather than STI damage. For context, IVF pregnancy rates per cycle run about 27 percent in Europe and 36.5 percent in the United States. Surgical outcomes tend to be best for women under 35 without other fertility issues.
For men with bilateral epididymal blockage from gonorrhea or chlamydia, the obstruction is often permanent even after the underlying infection is cured. Surgical sperm retrieval combined with IVF may be an option, but prevention through early detection and treatment remains far more effective than trying to reverse established damage.
In men, some fertility effects are reversible if caught early. Sperm DNA fragmentation caused by chlamydia has been shown to improve after antibiotic treatment, and semen quality can recover once acute epididymitis resolves, provided chronic scarring hasn’t set in.