Prime Numbers, Gestation, and Will

This George F. Will column, Questions for Sen. Schumer, ends with a question: On another matter, in Roe v. Wade, the court said that a privacy right -- an "emanation" of a "penumbra" of other rights -- guarantees a right to abortion, but also said that right changes with each trimester of a pregnancy. Does it seem at all odd to you that the meaning of the Constitution, or at least of its emanating penumbras, would be different if the number of months in the gestation of a human infant were a prime number?

I've been puzzled by this question for 24 hours. Prime number? I take it here that Will is saying something like this: "the gestation of a human infant" takes approximately nine months, so each trimester is three months. But if the "gestation period" was eleven months (and eleven is a prime number), then no one would talk about trimesters and thus Justice Blackmun would have reached a different conclusion in Roe. Is that it?

Now, I'm no mathematician, but . . . can't a prime number be divided by three? Now, granted, the result won't be a whole number or an integer, like three. Let me try this on my calculator, just to make sure. Hmm. Eleven (a prime number) divided by three equals . . . 3.6667. That's three and two-thirds for the decimally challenged. So even if the period of "gestation of a human infant" were eleven months, a prime number, someone might still talk about "trimesters."

In fact, isn't nine months really an approximate length? I've also heard that the "gestation period" (egad--is Will some kind of amateur zoologist now?) is really 40 weeks. But that doesn't divide evenly by three (13.333).

So I'm puzzled by the question. What does the prime number thing have to do with it? Is Will just trying to show off his "erudition"? Wow, is he smart, or what?