Chlamydia and gonorrhea are the two STIs most directly linked to infertility, in both women and men. They cause damage primarily by triggering inflammation in the reproductive tract that leads to scarring, and the risk is highest when infections go undetected and untreated. Several other STIs, including Mycoplasma genitalium, may also play a role.
Chlamydia: The Most Common Culprit
Chlamydia is the single biggest STI-related threat to fertility, largely because it so often flies under the radar. An estimated 77% of chlamydia infections never produce symptoms. That means the vast majority of people who have it don’t know they’re infected, and the bacteria can quietly damage reproductive tissue for months or longer.
In women, the estimated average duration of an untreated, asymptomatic chlamydia infection is more than one year. During that time, the infection can spread upward from the cervix into the uterus and fallopian tubes. About 10 to 15% of women with untreated chlamydia develop pelvic inflammatory disease (PID), a serious infection of the upper reproductive tract. But even without a full-blown PID episode, chlamydia can cause what’s called “silent” infection in the upper genital tract, damaging the fallopian tubes without any noticeable symptoms at all.
In men, chlamydia can infect the epididymis, the coiled tube behind each testicle where sperm mature. If left untreated, this can cause scarring that physically blocks sperm from reaching the semen. Studies consistently show that men with chlamydia have significantly lower sperm motility, reduced sperm concentration, and more abnormal sperm shapes compared to uninfected men. The encouraging finding: treating the infection significantly improved sperm morphology and reduced certain defects in studies that tracked men before and after antibiotics.
Gonorrhea and Reproductive Damage
Gonorrhea works through similar pathways but tends to be more aggressive. Like chlamydia, it can trigger PID in women and epididymitis in men. About 45% of gonorrhea cases never produce symptoms, so many infections still go unnoticed, though gonorrhea is somewhat more likely to cause obvious symptoms than chlamydia.
The consequences for male fertility can be severe. One study found that two years after a gonococcal infection, only 21% of men who were already fathers and 40% of the overall group had normal semen parameters. Gonorrhea damages male fertility through multiple routes: it can suppress sperm production, cause obstruction in the reproductive tract through infection of accessory glands like the prostate and epididymis, and trigger an immune response that floods semen with white blood cells, increasing oxidative stress that harms sperm.
In women, the fallopian tube damage from gonorrhea-related PID is identical to what chlamydia causes, and co-infection with both bacteria is common, compounding the risk.
How PID Leads to Infertility
Pelvic inflammatory disease is the critical bridge between an STI and permanent reproductive damage in women. When bacteria reach the fallopian tubes, the body’s inflammatory response creates scar tissue and adhesions. This scarring can partially or completely block the tubes, preventing an egg from traveling to the uterus. The inflammation also destroys the tiny hair-like cells (cilia) lining the inside of the tubes. These cilia are essential for moving the egg along; without them, even an unblocked tube may not function properly. The result can be infertility, ectopic pregnancy (where a fertilized egg implants in the tube instead of the uterus), or chronic pelvic pain.
The damage is cumulative. After a single episode of PID, up to 12% of women will have blocked fallopian tubes. After two episodes, that number jumps to more than one-third. After three episodes, up to 75% of women will have tubal blockage. This is why early detection matters so much: each untreated or recurring infection dramatically raises the stakes.
Mathematical modeling of chlamydia infections suggests that about half of expected PID cases develop within roughly 228 days of infection. That timeline means there’s a real window where screening and treatment can catch an infection before it causes irreversible harm, but that window closes with time.
Other STIs Linked to Infertility
Mycoplasma genitalium (often called Mgen) is increasingly recognized as a fertility threat. The CDC notes that untreated Mgen can cause PID in women, with the same downstream consequences: tubal scarring, ectopic pregnancy, and infertility. Mgen is not yet part of routine screening panels, which makes it easier to miss.
Trichomoniasis and other organisms in the vaginal microbiome may also contribute to tubal damage and infertility, though the evidence is less definitive than for chlamydia and gonorrhea. HPV, when present alongside chlamydia, appears to worsen sperm quality in men more than chlamydia alone, with significantly lower motility and more abnormal sperm forms in co-infected individuals.
Can Treatment Restore Fertility?
Antibiotics are highly effective at clearing the active infection, but they cannot undo structural damage that has already occurred. Once scar tissue has formed in the fallopian tubes or epididymis, antibiotics won’t reverse it. This is the core problem: treatment prevents further damage but doesn’t repair what’s already done.
For men, there is more room for recovery. Studies show that treating chlamydia can improve sperm morphology and reduce certain defects, suggesting that some of the damage to sperm quality is reversible once the infection is cleared. Permanent blockage from epididymal scarring, however, may require surgical intervention or assisted reproduction.
For women with chronic inflammation of the uterine lining (a condition called chronic endometritis, often caused by STIs), some research shows that successful antibiotic treatment can improve implantation rates and live birth rates. But results are inconsistent across studies, and for women with significant tubal scarring, fertility treatment like IVF, which bypasses the tubes entirely, is often the most viable path to pregnancy.
Why Screening Is Critical
Because the infections most likely to cause infertility are also the ones most likely to be silent, routine screening is the primary line of defense. Current CDC recommendations call for annual chlamydia and gonorrhea screening for all sexually active women under 25, and for women 25 and older who have risk factors like new or multiple partners. Pregnant women under 25 should be screened at the first prenatal visit, with repeat testing in the third trimester.
Men who have sex with men are advised to screen at least annually, or every three to six months if they’re on PrEP, have HIV, or have multiple partners. There are no routine screening guidelines for heterosexual men at average risk, which is one reason male STI-related infertility often goes unrecognized until a couple has trouble conceiving.
Anyone treated for chlamydia or gonorrhea should be retested about three months after completing antibiotics to catch reinfection, which is common and resets the clock on potential reproductive damage.
The Asymptomatic Problem
The single most important thing to understand about STI-related infertility is that you almost certainly won’t feel it happening. Roughly 77% of chlamydia cases and 45% of gonorrhea cases never produce symptoms. Among untreated infections, the overwhelming majority (95% for chlamydia, 86% for gonorrhea) went untreated specifically because the person never had symptoms, not because they avoided medical care. The infection was invisible to them.
This means that by the time infertility is discovered, often years later when someone is trying to conceive, the window for prevention has long closed. The damage accumulated silently, one inflammatory response at a time, in a reproductive system that showed no outward sign of trouble. Screening remains the only reliable way to catch these infections before they leave a permanent mark.