GOP presidential candidate Newt Gingrich has come under fire for his comment that human life begins at implantation, not fertilization. As a scientific matter, he's plainly wrong. Perhaps he's been confused by a separate debate: when pregnancy begins.
Much ink has been spilled on the definition of pregnancy. The debate has little or no practical importance, because pro-life advocates are always going to oppose the deliberate destruction of human life, whether or not that destruction also "terminates a pregnancy." For instance, there is clearly no pregnancy involved in embryonic stem cell research, which pro-life advocates almost universally oppose. Moreover, due to limitations in the current state of medical technology, most women do not become aware of their pregnancies until weeks after both fertilization and implantation have taken place, making it a moot point in the context of abortion. Nevertheless, it is an interesting debate, because it serves as a proxy for pro- and anti-abortion attitudes about pregnancy.
Pro-lifers tend to see pregnancy as a natural stage of life. It is that part of our lives which we spend in the womb, from conception to birth. From this perspective, it seems bizarre to claim that a woman can be carrying a genetically distinct human being in her womb, yet not be pregnant. That is the result, for a few days, if you believe that pregnancy begins at implantation.
By contrast, hard-core abortion supporters tend to view pregnancy as an egregious invasion of women's bodies. As an example, consider the pregnancy analogy drawn in Judith Jarvis Thomson's well-known essay "A Defense of Abortion," in which physical attachment to the body is what allegedly justifies the murder of the famous violinist. When pregnancy is viewed as an invasion rather than a natural part of life, it's no wonder that abortion advocates define pregnancy as beginning at implantation.
When an abortion advocate engages you on the topic of when pregnancy begins, recognize it as a side issue and redirect the conversation to the core question: when human life begins. And when you do, be sure to know your science better than Newt Gingrich does!