Information About Twins

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Categories Different Gender, Fraternal, Identical, Multiple Types, Pregnancy, Same GenderTags , , 9 Comments

*Note: There will be some tasteful “Birds and the Bees” talk on this blog post. If you are not comfortable with this, please do not read further.*

Twins in a Nutshell

When someone finds out I am pregnant, there are usually lots of congratulations: “Oh, that is wonderful! You are going to love it! You will be such great parents!”

Then they find out that we are going to have twins, and the mood tends to change: “Oh. Get ready to have your hands full!” or “I have a cousin who had twins,” or “Get your rest now,” or “Double trouble.”

When we first found out it was twins, my reaction was very much like those that we face every day. I was terrified. My perfect image of being a mom of my son or daughter and then having another little one a few years down the line… well that was gone. Could I go to the grocery store ever again? Would I need a bigger car? What happens when both of them cry at once? How can I do this?

Then, something happened, and I realized how unbelievably blessed we are to have not just one baby, but two. There is a reason why we were given this gift at this point in our lives, whether we thought we were ready for it or not.  So now, my reply to those Debbie Downers is “We are so excited to have twins! We are ready for this adventure.” Once I passively confront the negativity, it helps them change their mood too… usually.

Then the typical 2nd question comes: “Do twins run in your family?”

As I have answered this question about 100 times (and remember, I am 30 weeks pregnant at this point), I realize that so many people do not understand how twins “happen,” the differences between the different kinds of twins, and how it runs in families. I thought I might take this post to answer some of these questions.

What is an identical twin?

An identical twin is when one egg is released and is fertilized by one sperm. It separates into two different embryos, but they have originally come from the same egg and sperm. That means that they will have the exact same DNA. That also means that they will be boy/boy or girl/girl twins. There cannot be identical boy/girl twins, except in very rare cases of shared chromosomal abnormalities. They will look exactly the same (with minor differences due to “nurture” or development, but the “nature” is identical).

What is a fraternal twin?

A fraternal twin is when there are two eggs that are released during ovulation. They are both fertilized with two separate sperm. Genetically, these twins are no more similar than non-twin siblings. The only thing more than siblings that fraternal twins share are a birthday and a womb at the same time. Fraternal twins can be a boy/boy, girl/girl, or a boy/girl.

Due to the prevalence of fertility drugs and treatments that stimulate the release of eggs, the number of cases of fraternal twins is on the rise. Naturally, usually only one mature egg is released at ovulation. However, with fertility medicine, it causes more than one egg to be released at ovulation. With IVF (in vitro fertilization), more than one fertilized embryo can be transferred into the woman’s uterus. Although the release of multiple eggs can and does happen naturally, and identical twinning can occur with fertility treatments just as in spontaneous conception, twins from fertility treatments are usually fraternal.

Do twins run in your family?

Ah, the question that I know is coming upon the mention of twins. The answer that we give to these people is, “Yes. They run on both sides. We always joked about having twins, but we never thought that it would actually happen.”

But here is the real answer. Yes, they are FOUND in our family. My maternal grandfather was a twin (no surprise to any Doyle Dispatch blog readers as I talk about Papa Alan all the time). They are also found on Tim’s maternal side. However, here’s the thing: both of these cases are identical twins. Are you ready for this bombshell? Identical twins don’t “run in the family.” If you think about how identical twins form, it is the separation of an embryo. It is, in essence, a freak of nature. A really scientifically cool freak of nature, but a freak of nature, nonetheless.

Fraternal twins are actually the ones that can “run in the family,” and only on the mother’s side. For fraternal twins to be formed spontaneously, mom has to simultaneously release two eggs. However, we don’t have any fraternal twins in our recent family history. For us, it was just a fluke. But it was one that we are so excited to have!

What about the other kinds of twins I hear about?

In the twin world, it actually does get a bit more complex. There are different kinds of identical and fraternal twins, and their health and development in utero is tied to these differences. I will do my best to explain the differences here. If you are satisfied with the answers I gave above, please feel free to stop reading this section now.

Monozygotic Twins (MZ)

Also called identical twins. “Mono” = one. “Zygote” = egg. This is the “header” word for many of the following terms.

Monochorionic-Monoamniotic (Mo/Mo)

Identical twins that develop in the same inner and outer sacs.

Monochorionic-Diamniotic (Mo/Di)

Identical twins with one outer sac (chorionic) and two inner sacs (each embryo has its own amniotic fluid and sac). Both mm/mo and mo/di twins frequently share a single placenta. There are rare cases where fraternal twins have a fused placenta, but that is very unusual.

Dichorionic-Diamniotic Twins (Di/Di)

Two external sacs (chorions) and two internal sacs (amnions) to house the amniotic fluid. The Doyle Twins are Di/Di twins.

Twin Sketches

Conjoined Twins

These are identical twins where the division of the embryo starts, but it doesn’t finish. Often, conjoined twins will share organs.

Chimeras

It is possible, but enormously rare, for fraternal twin embryos to fuse early in development, resulting in a single person who has two people’s DNA. Chimeras usually go through life without ever knowing that theirs could have been a twin birth or that they have two sets of cells with different DNA.

So, that is “Twins in a Nutshell.” I hope that it has answered some questions for you. Leave a comment if you have questions or clarifications for me!

*This post originally appeared on Dory’s blog “Doyle Dispatch.” To read more posts about Dory’s pregnancy and nursery decorating on her blog, you can see the list here.*

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Zygosity: Do you know? Do you care?

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I spent part of the weekend attending workshops about twins and twinning. (I intended to post something immediately when I got home, but life got in the way and I’m a few days late). I found it quite fascinating to hear about what scientists know and don’t know about how twins are created. It was equally interesting to discover what twins have taught us about the joint influence of genetics and environment. One of the recurring themes of the workshops focused on different types multiples.

Here’s a quick review:

  • Monozygotic (MZ) multiples (can be twins or higher-order multiples) were created when one fertilized egg split to form two or more embryos.
  • Dizygotic (DZ) or trizygotic (TZ) multiples were created when two or more eggs were fertilized.
  • Triplets and higher-order multiples can be any combinations of monozygotic and di/trizygotic.

Monozygotic multiples are commonly known as “identical” and di/tri/quadzygotic are known as “fraternal.” These terms can be somewhat misleading as they suggest that monozygotic multiples are the same in every way. The truth is that the moment the egg separates, the two eggs cease to be identical and they are then influenced by different internal and external conditions. As a mother of monozygotic twins, I can tell you my daughters look very similar, but they are unique individuals and should be treated as such.

Anyway, back to the workshops, I learned a lot of interesting trivia about multiples. Did you know:

  • that monozygotic twins have more variation in birth weight than dizygotic twins, but by age 10 monozygotic twins are closer in weight and height than dizygotic twins?
  • there are more sets of female monozygotic twins than male sets, and more sets of female conjoined twins than male sets?
  • for most cancers, the risk of getting cancer isn’t any higher even if your twin has it
  • even though monozygotic multiples aren’t supposed to run in families, there are some cases where it seems to recur in families or to occur more often than expected in families with dizygotic multiples
  • language delays and learning disabilities seem to be more common among multiples
  • dizygotic twins may run in families and may be passed down by both men and women
  • there are mirror image twins where some traits are opposites (one is left-handed and one is right-handed, one has hair that parts on the left and one on the right, etc)

Since dizygotic twins run in both my family and my husband’s, we assumed that the twins we were having would be dizygotic. When they were born, the doctor said he thought they might be monozygotic and sent the placenta for testing to confirm his theory. If he hadn’t taken that step, we may have continued to assume they were dizyogtic even though they look very similar. I’m not sure if having that information makes a difference to us, at this point. Many families go for years without knowing the zygosity of their multiples, and apparently 75% of them are right about their assumptions.

Do you know for sure the zygosity of your multiples? How did you find out? Why is it important for you to know?

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