Why does your fertility decrease when you have chlamydia?

Tubal damage. A chlamydia infection can damage fallopian tubes by way of adhesions or scar tissue formation. This prevents fertilization by blocking the egg and sperm union.
Tubes+ maybe ovaries. I agree that tubal damage and/or scarring around the tubes or ovaries from chlamydia or pelvic infalmmatory disease is the major problem. Even women with a normal hsg may be at greater risk of ectopic (tubal) pregnancy as well. Some women with tubal damage from chlamydia may also have low egg supply (diminished ovarian reserve) when checked carefully: ultrasound, amh and Clomid (clomiphene) challenge tests.
Tubal damage. The main impact (and it can be extremely significant) of chlamydia on female fertility is tubal damage. This includes adhesions ('scar') around the tubes and/or ovaries, thickening of the tubal walls and disruption of the cilia that line them, to partial or complete blockage. A hysterosalpingogram (hsg) is often the first step in evaluating tubal damage, whether from chlamydia or other causes.
Tubal damage. After chlamydia infection, you will develop pelvic inflammatory disease. It will cause tubal damage and scar tissues in the pelvis which will significantly reduce the tubal function. In addition to infertility, patient may also at increased risk of tubal pregnancy.