The concept I describe most frequently to anyone interested in fertility is that of ovarian reserve or the "egg factor."

As of this writing we are quite capable of bypassing the problems of poor quality sperm or low sperm count or problems stemming from dysfunction or disease of the female reproductive system such as endometriosis or tubal disease. Still vexing though are women in the reproductive age group whose eggs do not seem to respond to the tools that we have available in 2001.

Eggs are not replaceable. Women are born with all of the eggs they will ever have, and the number of eggs rapidly depletes as women age. Even before a first menstrual period, the number has shrunk from the millions present just before birth to the hundreds of thousands, and many more are lost monthly. Each menstrual cycle sees hundreds of eggs start the journey to maturity, a journey that only one or two will successfully complete. Each woman's ovaries have their own rate of egg depletion, a sort of ovarian career that lasts from puberty to sometime before menopause.

Over time, the chance of conceiving in any particular month drops. For years we debated whether this decline was a result of the aging ovaries or the aging uterus, but the discovery that a woman's reproductive potential could be greatly increased by the use of donated eggs demonstrated that the eggs and not the uterus were the cause of the decreased fertility. Even among women of similar ages, though, not all eggs are equal in their ability to turn into a successful pregnancy when matured and exposed to adequate amounts of sperm at the proper time.