There was this article from a couple of years ago: http://www.washingtonpost.com/wp-dyn/content/article/2010/02/22/AR2010022203639.html

"Whether you are aware of your incessantly ticking biological clock or not, the absolute last thing that any woman of steadily advancing childbearing age wants to hear when she flips on the morning news shows is: Women lose 90 percent of their eggs by age 30. <snip> Using a mathematical model and data from 325 women, the researchers found that the average woman is born with around 300,000 eggs and steadily loses them as she ages, with just 12 percent of those eggs remaining at the age of 30, and only 3 percent left by 40.

Also, as another data point, it is possible that the recent change in your "will to parent" might reverse polarity again once you hit your late twenties and early thirties. I've been amazed at this change that has sort of crept up on me over the last few years - from wanting "at least three" in my early to mid twenties to finding the whole enterprise rather frightening (in terms of lifestyle changes and sacrifices needing to be made). It's possible I'm completely atypical since there are no shortage of stories of women in their thirties becoming even more desperate for children.