Rachel Downtain is a telecommunications project manager who says her friends would describe her as tall, slender, fit and active. Not someone you'd think would fail to find a mate. Yet, of late, Ms. Downtain has been sifting through sperm-donor Web sites. This is not her first choice for how to start a family, but at 35 she says she's quickly running out of options.

Ms. Downtain's story should sound familiar. In recent months the spike in college-educated women deciding to have a husbandless family...