There is currently no legal limit to the number of children that can be fathered by one donor. Thus, for example, as more women have chosen to have babies in this way, the chances have increased that their offspring could unknowingly become romantically involved with a half-sibling. There is also the fear that illnesses could be spread faster through the population. The American Society for Reproductive Medicine has recommended only 25 such births per 800,000 people, while other countries, such as France, England and Sweden, have already placed restrictions on the industry.

In the meantime, parents are taking matters into their own hands. Many tell their children to memorize their donor's confidential identification number so they can make sure they're not dating a sibling. In 2000, the Donor Sibling Registry was created so that children of donors can connect with each other, or simply see how many half-siblings they have. "Family," it seems, has an ever-changing definition -- but is that always for the better?