Identical versus Fraternal Twins

You can imagine how often we get this question: "Are they identical or fraternal?" now that we know they are both boys (although, surprisingly, some people ask the same thing of boy/girl sets. Duh, people). Then, "Do they run in your family?"

Some people who ask are just trying to determine if we used fertility treatments or not because most babies helped along with drugs or IVF are fraternal. Other people are genuinely interested.

The answer is that we don't know. But I've learned a lot about the differences between the two types of twins.

Identical twins occur when one sperm fertilized one egg. That zygote then splits into two, creating two eventual babies with the exact same genetic make-up (but differing environmental influences throughout their lives affect which genes are switched on or off, if that makes sense, so identical twins aren't the exact same person). Identical twins, according to the experts, are a random occurrence. In other words, identical twins do NOT run in families. They occur at a rate of about 1/250 and every woman in this world has the exact same chance of having identical twins. The zygote can split at different times. If it splits very early, identical twins can have two placentas. If it happens later, they have just one placenta. If there is just one placenta, you know you have identical twins, as fraternal twins always have their own.

Fraternal twins, on the other hand, occur when two sperm fertilize two eggs. Without fertility treatments, a woman's chances are about 2/100 to have fraternal twins when she is younger than 30 and higher when she's over 35 (since her ovaries are having some sort of going out of business sale and just drop like crazy). The rate also increases significantly if the mother herself is a fraternal twin. Rates also increase when certain drugs are used, such as Clomid, which stimulate egg production, or when multiple embryos implant during IVF treatments.

So, in the fraternal scenario, a woman must release two eggs in one cycle for this type of twinning to take place. Dropping two eggs in one cycle can be a genetic trait - hence, fraternal twins DO run in families. And it comes ONLY from the mother's side in any given father/mother relationship. Having said that, Ryan could carry the gene for dropping two eggs but considering he doesn't actually do any egg dropping himself, he can't determine twins in our relationship - but he could pass it to any daughters we might have (hence, the whole theory of twins skipping a generation, which is only true when it comes through the father's side). Fraternal twins always have two placentas.

Since we are having two boys who each have their own placenta, they could be identical or fraternal. Given that I do have twins on my side 3 generations ago and given the fact that fraternal twins are so much more common, our doc says our chances are 80% that they are fraternal.

How will we ever know? Well, they could look very much unalike. But even if they look very much alike, they could be fraternal (Mary Kate and Ashley Olsen are NOT identical, for example). DNA testing is the only way to be really sure. Either way, since I'm younger than 30 and we used no fertility treatments, we should play the lottery more often.

Since Ryan and I have similar coloring, we're banking on their height telling us if they might be non-identical. I mean, if one comes out at 22 inches and the other at 17, you just know has Haas genes and the other Doran genes. No DNA testing needed there, that's for sure.

Hope this was super enlightening.