I thought the doctors would be able to tell us whether my wife was carrying fraternal or identical twins. When she was pregnant with them. Wasn’t that something they could just know? Well, the answer was “not always”.

There are several ways of immediately identifying twins as dizygotic – that is, as coming from two separate eggs, aka fraternal – the most obvious being boy/girl. In our case, boy/boy, so that didn’t help. By the time of their birth, all other factors which could have marked them as fraternal were eliminated: placenta, inner amniotic and outer chorionic membranes, and same blood type. But what about separate sacs, don’t identicals share the same sac? Not for the super majority of monozygotes, and in fact it’s the rare same sac scenario that leads to such difficulties as conjoined twins. Thank goodness then, no confirmation there.

And for three years that’s what it boiled down to. No way of knowing, except that fraternals were statistically more probable. But in the back of my mind, every time a possibility of fraternal was eliminated, Bayes’ theorem of conditional probability made monozygosity more likely. To know for sure, we had to go to DNA. And we didn’t.

Because we didn’t really care. Except, everyone else did. “Are they identical or fraternal?” became a constant refrain. At first I tried explaining that we didn’t/couldn’t know. This was met with almost universal disbelief, and since I previously had to study up on the subject to convince myself it was unknowable in our case, the confusion was understandable. But it’s a minimum five minute conversation to explain the ins and outs of “why the doctors didn’t know” – and while that was acceptable for friends and family, the question was asked by almost everyone: parents of classmates, the clerk at HEB, other parents in the park.

That took up too much of my time, especially in light of the fact that chasing two little boys takes substantial quantities of both physical and mental energy. (Before they were even speaking, you could actually see the twin thought bubble: “Hey, let’s make a run for it. Daddy can’t catch both of us if we head in opposite directions.”) Plus the topic grew boring, so I just started answering the question with “fraternal”.

But then people started arguing with me. On the playground, wherever we were. “Are you sure?… They look so much alike.” So I varied the answer at times, and finally settled on “identical” as the reply because it required no extra explanation. But it felt a little weird either way, since “don’t know” was the honest answer.

Reading Crime & Federalism/Mike’s post “Procreating Is Narcissism” reminded me of my promise to tell the story of why we didn’t know, at least up until the point when the DNA said they were clones. The twins will be four this summer, and the chorus of “Are you going to have more children?” is hopefully entering its last phase. But I’ve got a new answer for people who ask me that, and it’s a true story.

I took the boys to the Austin Children’s Museum, where I ran into a father of a nine year old girl and twin eight year old identical brothers. Most of our conversation consisted of me asking him to lie and just to tell me that it gets easier, which he refused to do, but he left me with this line that I now use when asked about the possibility of further procreation:

“You often see twins with an older sibling… but you rarely see twins with a younger sibling.”

Never thought of it that way before, but it makes a great answer that everyone can understand.