Yesterday's Globe carried an item that Colorado is considering adopting a measure which would define a legal human life as beginning at conception. Questions around reproductive ethics and law raise strong emotions, and I won't attempt to argue either one of them. However, law & ethics should be decided in the context of the correct scientific framework, and that is what I think is too often insufficiently explored.
Defining when life "begins" is often presented as a simple matter by those who are proponents of "life begins at conception" definition. However, to a biologist the definition of conception is not so simple. Conception involves a series of events -- at one end of these events are two haploid cells and at the other is a mitotic division of a diploid cell. In between a number of steps occur.
The question is not mere semantics. Many observers have commented that a number of contraceptive measures, such as IUDs and the "morning after" pill would clearly be illegal under such a statute, as they work at least in part by preventing the implantation of a fertilized egg into the uterine wall. Anyone attempting to develop new female contraceptives might view the molecular events surrounding conception as opportunities for new pharmaceutical contraceptives. For example, a compound might prevent the sperm from homing with the egg, binding to the surface, entering the egg, discharging its chromosomes, locking out other sperm from binding, or prevent the pairing of the paternal chromosomes with maternal ones (there's probably more events; it's been a while since I read an overview). Which are no longer legal approaches under the Colorado proposal?
At the other end, if we define human life by a particular pairing of chromosomes and metabolic activity, then when does life end? Most current definitions are typically based on brain or heart activity -- neither of which is present in a fertilized zygote.
Again, the question is not academic. One question to resolve is when it is permissible to terminate a pregnancy which is clearly stillborn. Rarer, but even more of a challenge for such a definition, are events such as hydatiform moles and "absorbed twins".
In a hydatiform mole an conception results in abnormal development; the chromosome complement (karyotype) of these tissues is often grossly abnormal. Such tissues are often largely amorphous, but sometimes recognizable bits of tissue (such as hair or even teeth) can be found. Absorbed twins are the unusual, but real, phenomenon of one individual carrying a remnant of a twin within their body. Both of these conditions are rare (though according to Wikipedia in some parts of the world 1% of pregnancies are hydatiform moles!) but can be serious medical issues for the individual carrying the mole or absorbed twin.
Are any these questions easy to answer? No, of course not. But they need to be considered.