This is coming a bit late, since last week Mississippi voters turned down the proposed amendment that would have established personhood for fertilized human ova. Let me pause for a moment in amazement at the creative potential of the law and the truly god-like power of voting. The right kind of ballot can create life!
In any case, one of the many angles of attack for writers organizing against the personhood amendment was against ignorance of scientific knowledge, and against the perceived misrepresentation of science, by supporters of the amendment. A key issue surrounding the amendment was whether it would ban hormonal birth control, and discussions of this issue, even among feminists and others opposed to the amendment, revealed a widespread belief that the pill works by preventing fertilized ova from implanting. In truth, as best scientists can tell, the pill’s main function is to prevent ovulation in the first place.
However, some amendment supporters were quite clear that they interpreted the amendment as outlawing not only abortion, but also many forms of contraception.
Indeed, at least one pro-Personhood doctor in Mississippi, Beverly McMillan, refused to prescribe the pill before retiring last year, writing, “I painfully agree that birth control pills do in fact cause abortions.” (via Irin Carmon, Salon)
On its Web site, Yes on 26 confirms that this rule covers some IUDs and oral contraceptives: “We are opposed to those birth control methods which act as abortifacients. These could include forms of the pill which act to prevent implantation of the newly formed human into the lining of the womb; forms of the IUD, which can act the same …” (via William Saletan, Slate)
Amanda Marcotte spent a fair amount of writing-time pointing out the ways in which these incorrect claims were being repeated by the amendment’s opponents. In their efforts to publicize the real possibility that a personhood amendment, in practice, would likely be used to curtail access to birth control, opponents either didn’t realize the mistake, or didn’t want to get bogged down in technical arguments about biology.
Politically speaking, avoiding the tedious biological discussions of how the pill works and the tedious legal discussions of the interaction of science and policy and going straight to what the audience needs to know—anti-choicers are trying to ban the pill!—is the smart move. I don’t blame anyone for going there. […] But still, it bothers me. Every time we fail to address the blatant lie about how the pill works at the center of this debate, we allow the lie to linger. (via Amanda Marcotte, HR Reality Check)
So are fertilized-egg-personhood supporters anti-science? On this level, maybe that would be a fair criticism. The research we have best supports the notion that the pill prevents ovulation, not implantation. Claiming that the pill is essentially an abortifacient is not accurate.
On another level, though, this amendment depends on a world of science and advanced medical technology. Even to ask whether life begins at conception requires a specific biomedical understanding of human reproduction on a microscopic level. You need a theory of cells, of eggs and sperm, even of DNA. This diagram needs to make some sense to you:
Diagram of egg and sperm mutually engaged in fertilization
It helps to have really high-tech ways of viewing and recording images of tiny cells. Indeed, it turns out that the advent of ultrasound and other imaging technologies have played an important role in allowing us to develop earlier notions of personhood.
Personhood is essentially a social act: it is the recognition by a community that a particular entity is also a person. Who gets to be a person, how, and when–these can vary dramatically from one place to another and across history. (Over-simplifications ahead!) Janet Carsten, for example, writes about a group of Malay people who understand personhood as intimately related to kinship: to be a complete person, one must be in a kinship network. Kinship develops over time through a gradual process of eating and living together with others.  Nancy Scheper-Hughes famously described the contingent personhood of children in a Brazilian favela; lacking the resources (economic, emotional, and otherwise) to care for their children, many mothers delayed attributing personhood to children until they seemed likely to survive.  The US Constitution ascribed personhood from birth to enslaved Africans, but counted them as merely 2/3 of a full person. (That last 1/3 included rights to vote, marry, travel and work freely, etc.) (One wonders what kind of rights an embryonic person would have?)
The point being, of course, that there’s nothing objective or obvious about what personhood is or when it begins.
Nonetheless, a significant trend in the US has been for the beginning of personhood to shift earlier as our ability to see into the womb has improved. Furthermore, as technology has enabled us to think more (and see more) about the early stages of gestation, the treatments and procedures that have become possible have forced us to make decisions about personhood, as Rayna Rapp discovered in her research into amniocentesis. Once it becomes possible to test for Down Syndrome, she writes, one has to make a choice about whether to test. And then, what to do with the results. 
Modern biological science and the technology of medicine make it possible to think about personhood for a cell, an embryo, or a fetus.
Anti-science depends on science?
Well, of course it does, just like any opposition movement. But the personhood debate is quite telling of the deep influence biomedicine has had on US culture that even those whose views are often believed to be anti-science, and who themselves may advocate a stronger role for religion rather than science in public life. The very position taken by religious conservatives is one that science has only recently made thinkable.
 Carsten, Janet 1995. “The substance of kinship and the heat of the hearth–feeding, personhood, and relatedness among Malays in Pulau Langkawi.” American Ethnologist 22(1): 223-241
 Scheper-Hughes, Nancy 1992. Death Without Weeping: The Violence of Everyday Life in Brazil. Berkeley: University of California Press.
 Rapp, Rayna 1999. Testing Women, Testing the Fetus. New York: Routledge.
Image of a zygote