How does smoking affect a women's fertility?

Low blood flow. Smoking causes arteries to clamp down, reducing blood flow to a number of organs. In women, the ovaries are effected by both the lower oxygen levels, reduced blood flow and things like carbon monoxide and formaldehyde in the smoke. Pregnancy is tremendously affected as well, with smaller babies, placentas that are not normal, and many other problems. Quitting will protect against all of these.
Decreases fertility. Smoking is particularly harmful to fertility; women who smoke have been shown to go through menopause earlier in life, and to have less success with infertility treatments. Smokers underoing in vitro fertilization are half as likely to conceive as similar age women who do not smoke. The other chemicals in the cigarettes are damaging to the eggs which increases the risk of infertility.