Bullshit most foul! I would call on you, dental technician, had you not just inserted an unwieldy piece of cardboard into my mouth and retreated to your safe space to x-ray my mouth.
Upon learning that I have identical twin boys, she reported that, "identical twins run in my family. On both sides."
Here we go again. At some point I will un-appoint myself from the position of one-woman twin educator. I'm starting to think that day has arrived.
"Actually," I said, between cardboard insertions, "identical twins don't run in families. They're always a fluke."
"Really?" she said. "Weird. They're definitely on both sides. Although I think the one set might not be identical because they're boy/girl."
Riiiight. The one myth I'm always surprised still needs to be debunked. Boys and girls cannot be identical. They might have the same color hair and the same color eyes. They might be the same height and weight. They might like the same things. But at that first diaper change....whoa! Something's definitely different!
So, if boy/girl twins cannot be identical, all the celebrity twins you've heard about are not identical. Angelina Jolie, Julia Roberts, Jennifer Lopez, Mariah Carey, and Neil Patrick Harris all have girl/boy twins.
The rate of identical twins has remained the same since anyone thought to record the incidence of such things. If anything, it has gone down as testing has become more precise than just "they look a lot alike." Did you know that the Olsen twins are not identical? They look very much alike and are the same age, but they do not have the same DNA.
The more official term for identical twins is monozygotic, meaning that they developed from a single fertilized egg. For reasons no one understands yet, the egg splits at some point. The twins can be 'less identical' if the egg split earlier in the development, or, at the extreme, conjoined, which occurs when the egg splits so late in development that the split is not complete.
My own twins developed due to a split that happened somewhere between those two extremes. In utero, they had their own amniotic sacs, but they shared one placenta. When the egg splits earlier, the fetuses might have separate amniotic sacs and separate placentas. They might not look as much alike as other monozygotic twins and might require a DNA test to know for sure. Since my twins had one placenta, we did not need to do the testing. There is no way for two eggs developing side by side to create one placenta.
There is no way to influence the likelihood of having or not having identical twins.
Fraternal twins occur when two eggs are fertilized. These fetuses will always have separate amniotic sacs and separate placentas. They will be as alike as any other two siblings from the same parents except that they are the same age. However, they might have greater similarities due to sharing the embryonic environment and having more closely corresponding childhood developmental situations than siblings who are a few years apart. The rate of fraternal twins has increased dramatically in recent years for a couple of reasons.
A lot of women are having babies at an older age and older women are more likely to 'drop' an extra egg. Maybe this is an evolutionarily adaptive way to increase gene output before the system shuts down altogether.
The use of fertility drugs increases the chances of having twins or other multiples. The use of fertility drugs has gone up due to their increased availability and safety and possibly also to women waiting to have babies and then having a harder time getting pregnant.
IVF (in vitro fertilization) is a major cause of the increase in fraternal twins and, I suspect, the agent behind the celebrity twin boom. Multiple fertilized eggs are inserted into the mother's uterus and then there is a wait to see how many will implant. Responsible doctors don't generally insert a large number, but then you have some, like the one responsible for the 'octo-mom,' who seem willing to go to extremes to guarantee baby. Anyone doing IVF, if they attempt to implant more than one fertilized egg, has to be prepared for twins or more. By the time the prospective parents have gone through all the rigmarole required for IVF, they generally want to take the best odds of getting a baby and not have to go through the cost and pain of the process again. This is played against the odds of winding up with more than one baby. And people who are going to all this work to have at least one are generally not put off by the prospect of twins and in fact might be thrilled by getting 'two for one.'
Naturally occurring fraternal twins CAN run in families although this method of twinning is not responsible for any increase in the rate of twins. And now to the misinformation about twins running on "both sides of the family." While you might indeed have twins on both sides of the family, this does not affect your own chances of having twins. Sorry guys, but this all happens on the woman's side of things. A woman can inherit the tendency to release more than one egg during ovulation. She can inherit this from her mother, from the female line. If you think about it: the male DNA doesn't need a spot for directions on egg-releasing during ovulation, does it? Thus, this explanation for why Angelina Jolie had twins makes no sense: "Well, you know, Brad's sister had twins." If Brad's sister had naturally-occurring fraternal twins, she could have inherited that tendency from her mother or grandmother. And she could pass it down to her daughters. But she wouldn't share that with Brad, and even if she did, his sperm (miraculous thought they may be) could not coax an extra egg down Angelina's Fallopian tubes. We presume that Angelina does not share DNA with her husband's siblings so that history is not relevant. We would need to look at the female line of Angelina's family to see if twins run in the family.
My brother and his wife recently had a set of fraternal twins which caused several people to say to me: "Wow, twins really do run in your family!" Well, it is lucky and unusual to have two sets of boys in one family, but they are not connected. First, my twins are identical, an unexplained fluke. Second, I am not genetically related to my brother's wife so even if I carried some tendency toward fraternal twins, I would have no way to pass that to my sister-in-law as if it were the gravy at Thanksgiving.
I don't expect everyone to know all these facts. I learned most of this after becoming pregnant with twins when it all became extremely relevant. Because I was 40 when my boys were born, people often assume I used some sort of fertility assistance and will fish about for information. "So, do twins run in your family?" I will say no, which is true, but that confirms their suspicions. "But identical twins don't run in families," I add. "No one knows what causes an egg to split." Nine times out of ten, this leads to a conversation full of myths I feel the need to debunk and me imparting way more information than the person bargained for. Then, unless they truly paid attention, they probably still wonder if I used fertility treatments. Which is none of their business.
Now that I think about it, I should have waited until the dental technician emptied my mouth and simply said, "they run in my family too. In fact, my brother's wife just had twins....."