We are pleased to report to you the results of the twin zygosity test that you requested. Analysis of the DNA indicates that Elizabeth Rose and Coraline Ann are monozygotic, or more commonly referred to as identical twins.
We first isolated DNA from cheek cells from the swab kits that were returned to our lab. We amplified the DNA using a technique called PCR. Next, we analyzed 7-8 standard DNA markers (D5S818, D13S317, D7S820, D16S539, vWA, TH01, TPOX, and CSF1PO) and determined that the highly variable DNA markers were the same for both twins. Thus, there is greater than a 99% probability that the twins are monozygotic.
Thus part of the mystery surrounding the triplets is revealed. It turns out that when we conceived our triplets, originally Erin had just two eggs (probably because the fertility medicine she was on worked too well). These were fertilized as a boy and a girl, and within the first two days, the girl zygote divided into two embryos (any later, and the girls would have shared a placenta). This splitting does not seem to be a hereditary trait, but an anomaly that occurs in 3 out of every 1000 births. Of these, only 20-30% are diamniotic twins, like Betty and Coraline. So with these test results, it is finally confirmed that we have triplets and twins!
And a link, to an article that came out a month before they were born, possibly explaining how identical twins are formed:
"Study: Twins form after embryo collapses"