Age, STDs, smoking. Women's ovarian reserve (egg numbers and quality) starts to decline from mid 20's on average, and fastest by the 40's. Smoking, stds like chlamydia, endometriosis, surgery on the ovaries, and cancer chemo or radiotherapy reduce ovarian reserve faster. Chlamydia, pid, endometriosis or surgery can block the fallopian tubes or cause scarring around ovaries. Overweight or underweight reduces fertility.
Many things can. A woman's risk of infertility is affected by such things as her age, lifestyle choices (such as smoking, drug use, etc), sexually transmitted infections, hormone imbalances, prior surgeries on the uterus or ovaries, exposure to reproductive toxins, body mass index, presence of endometriosis, family history, and overall health.