When Spencer and Maeve came in to Megan's office that Saturday afternoon, a half hour after getting the call, they were worried and bewildered. What was so important that they had to know it right then? They were sure there was some kind of abnormality because if there wasn't she would've divulged the results over the phone instead of having them come in. The couple couldn't help but give their friend sheepish, pleading please tell us everything's ok looks.

"Hi guys, both of you please have a seat." Megan responded, gesturing toward the off-white all too-modern sofa against the large set of windows that took up almost the entire wall and looked out over the rest of the campus.

They obeyed.

"Megan, what is this about? What is it that you couldn't tell us over the phone…?" Maeve asked, her voice almost frantic. She tried to read Megan's expression but it was mixed.

"As you know, I finished analyzing the karyotype…" Megan began.

"And…?" Reid cut in. "What did it show? Is there something wrong with our baby?" he asked, fearing that one of those moments every parent dreads was in their imminent future. A moment when everything they had imagined, dreamed of, and looked forward to for the last four months, would all come crashing down and blow up in their faces.

"I brought you both in here instead of just calling you because if we didn't have this conversation in person you might not believe me…" The older woman said.

"What do you mean?"

"It's one of the rarest chromosomal combinations known to man… this is the first time I've seen it for myself, though I did read about it once in medical school…"

"Megan please…get to the point." Maeve demanded, as though she were being tortured.

"There is not one baby…there are two…a male/female pair of mono-zygotic twins."

"Wait a second…how is that even possible?" Reid asked. "Mono-zygotic twins are genetically identical…"

"Yes, and these two are right down to the fact that one of them is a boy and one of them is a girl. Other than that they are identical…"

"W-wow…" Maeve exclaimed. Being a geneticist herself she knew already what Megan was about to explain.

"Male/female identical twins are only born when something we call double-nondisjunction occurs on chromosome twenty-three, the XX or XY pairing, an egg which mistakenly contains two X chromosomes instead of just one is fertilized by a sperm that contains an X and a Y instead of one or the other… normally someone with two full sets of any chromosome isn't going to survive very long…but sometimes the embryo splits, creating a pair of twins…who end up saving each other's lives by existing. This is rare obviously… what is even rarer is the way in which they split… most of the time when this happens they end up with the girl having Turner's syndrome and the boy having Klinefelter…but your embryo split in such a way as to create a healthy, chromosomally normal pair. You guys just dodged a gigantic bullet… Remind me to take you two gambling sometime because you basically won the genetic lottery…"

"So…they…they're ok…?" Spencer asked, he was in emotional overload but right now the health of his child-children, was all he cared about.

"Yes, in fact had they both been one or the other instead of one of each, I probably wouldn't have even noticed…other than this the karyotype came out perfectly…no missing or transplanted sections, no other instances of nondisjunction…they are fine as far as the karyotype is concerned. A karyotype doesn't pick up everything, it's really more for the big-picture kind of stuff, and given what we know already happened, and just how rarely these things self-correct like that…I'm going to suggest that it might not be a bad idea to get a full DNA profile of the twins after they're born to look for single gene mutations that may have gone unnoticed…"

"Why can't we just do this now?" Maeve asked.

"No!" both Megan and Spencer replied in unison.

"Doing a karyotype once won't do much harm…but a second extraction could do real damage… at this stage it's going to be far safer to wait and swab them for DNA non-invasively as newborns than to stick another big needle in your abdomen…" Megan told her.

"She's right Maeve…"