How do you define twins?

An Arkansas woman is pregnant with two babies, but they are not twins, according to this MSNBC report. An ultrasound revealed that Julia Grovenburg was pregnant with a girl and was about eleven weeks along. Then the parents were shocked to discover another baby, a boy estimated to be about eight and a half weeks' gestation. The two babies are the result of superfetation, the term used to described conceiving while already pregnant. While rare, it can produce a multiple pregnancy where the fetuses display a marked difference in gestational development.

I don't agree with the statement that the babies aren't twins. Encyclopedia Britannica states that twins are "two young who are simultaneously born from one mother." That definition doesn't characterize them based on when they were conceived. However, that definition isn't entirely specific either, since no multiples are technically born simultaneously. They are always at least a few seconds, or usually several minutes, between the births. And some twins or higher order multiples even have different birthdays. Some are born days or even weeks apart due to delayed interval birth, when one multiple is delivered prematurely, but doctors are able to halt labor and keep the remaining multiple(s) in their mother's womb to grow and develop. Are they still twins? It is not clear whether the Grovenburg babies will be born "simultaneously"; their official due dates are in two different years: late 2009 and early 2010. However, they are both expected to be delivered together in December.

What do you think? Should the Grovenburg babies be considered twins? Please leave a comment and share your thoughts.

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