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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