The XX Factor: What women really think.

The Logic Behind Fertility Stats

Guest post by Sarah Elizabeth Richards, the author of "The Clock Ticker's Reprieve," a narrative account of how egg freezing affected the lives of five women, to be published by Simon & Schuster in summer 2010.

In response to my article about Rielle Hunter's chances of having a baby at 43, a reader asks how researchers determine a woman’s chances of becoming pregnant each month, especially when they don’t know when or how frequently she is having intercourse. Since a woman is only fertile about three days a month, the stat assumes she’s had sex during this window ... (Read more in DoubleX.)