Monday, September 12, 2011

One zygote or two? When it's hard to tell

Ultrasound is a wonderful thing, and we  know a lot about how twins deveop but there is one situation when even the experts are baflled - that is, when twins develop  at the same rate and in separate sacs.

In about 30% of cases, monozygotic (MZ) twins develop in separate sacs.  In 100% of cases dizygotic (DZ) twins develop in separate sacs.  The situation looks the same for both MZ and DZ twins on ultrasound, as I found when making the illustrations for Womb Twin Survivors : the lost twin in the Dream of the Womb.

Are these twins  from one zygote or two?

If  DZ twins turn out to be of the same sex, things get even more confusing.  For the twins, the need to know if they are identical or not is so great that they may resort to a DNA test to find out. MZ twins share all the same genes, but DZ twins don't. That is why they often look different from each other.  But they may be so similar they can be mistaken for each other, as long as they are not seen together, when the differences become apparent. The Olsen twins for example are DZ twins, but they are so alike they played the same part in the FULL HOUSE show and no one noticed!

So if you are a womb twin survivor and your twin was made from a different zygote from you but was the same sex as you, how can you tell? In the next series of blogs over next week, I will explore some of the indications - at least the ones I have found so far. I am sure there will be many more,  as research into this subject continues around the world.

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