Giving birth to reasoned discourse

PZ Myer's explanation of Plan B, a.k.a. the morning-after pill or emergency contraception, and the accompanying exchange of ideas generated by his post at Pharyngula, is the best example I have ever come across of what every blog aspires to be. If you have a free half-hour, and are even remotely intrigued by either reproductive biology or blogging in general, read the whole page.

First, the explanation of the science behind pregnancy and contraception is succinct and easy to understand for anyone with no more than high-school biology. Second, the comments that follow address the issues PZ chose not to. The questions asked and the answers supplied paint a most complimentary picture of the intellectual depth of Pharyngula's readership. There's discussion of research, religion, politics, feminism -- you name it -- almost all of it delivered in calm, reasoned tones.

Here's the introduction:

There has been an oddly evasive struggle going on in Washington DC for the last several years. We have a safe, easy method of emergency contraception that has been turned into a political football, with Republicans playing their usual role of criminally stupid thugs, trying to crush a simple idea: Plan B contraception. It illustrates exactly how the Religious Right is trying to intrude on your private life, and in particular, how they want to control women.

Of particular interest to me was PZ's claim that Plan B only prevents ovulation and does nothing to interfere with implantation of a fertilized egg. It did not slip past his readers without debate. I wasn't the only one to recall that, as William Saletan wrote a couple of days ago at Slate, the scientific community is not of one voice on the potential of progestin-based contraception, such levonorgestrel, to prevent implantation.

James Trussell, director of the Office of Population Research at Princeton University, offers a less absolute opinion:

Treatment with ECPs containing only levonorgestrel during the peri-ovulatory phase may fail to inhibit ovulation but nevertheless reduce the length of the luteal phase and total luteal phase LH concentrations; this observation suggests a post-fertilization contraceptive effect.

The consensus seems to be that even if this is true -- and it is by no means settled -- it would happen extremely rarely. Unfortunately for those who want black-and-white answers on just what the chances of EC preventing implantation really are, the only way we have at the moment to determine the fate of an egg prior to implantation is to open up the suspect mother.

More on that, a lot more, at Pharygula.