Re: [MLUG - DISCUSSION] Identical Twins

[Nathan Odle <EMAIL:PROTECTED>]

Vern Green wrote:

>My wife just had an interesting thought though. How do they know they
>are identical? 
>

Thus begins the geek discussion :)

I've learned a lot about this in the last 24 hours, needless to say...

There are 5(!) ways to have twins.  Here's an explanation of how they work:

First, a couple relevant pieces of information - babies (twins or no) 
are 'double-bagged'.  They develop within an inner membrane, called the 
"amniotic sac" (also "amnion").  This sac contains (surprise) the 
amniotic fluid and cushions the baby throughout the pregnancy.  They 
also are inside another membrane, the "placenta".  The placenta performs 
a lot of duties, but mainly it's a life-support system for the baby.  It 
is connected to the baby by the umbilical cord, and acts as filter in 
addition to other duties.

Also a quick definition or two: 
Zygote:  a fertilized egg
Dizygotic == Two Zygotes == "Fraternal twins" == Don't have the same DNA
Monozygotic == One Zygote == "Identical twins" == Have the same DNA

Ok, so on to the types of twins. 

1.  The first type is the "Fraternal", or Dizygotic twin.  This happens 
when multiple eggs are fertilized, and the babies develop together.  
Since they don't have the same DNA, they can be different genders.  This 
is also the type of twinning that runs in families, apparently some 
women have a genetic disposition to release multiple eggs at a time 
leaving the door open for multiple pregnancy.

The remaining four types all have the same DNA (are "Identical" twins) 
because they come from the same zygote, but are differentiated by the 
day of pregnancy upon which the ova splits. 

2.  Dichorionic Diamniotic Monozygotic Twins - 2 placentas, 2 amniotic 
sacs.  Occurs when the ova splits in <72 hours.  This is the safest form 
of twinning because they each have their own separate support systems.  
They have two placentas because the ova splits prior to the point at 
which the placenta begins to differentiate.  The babies' side of the 
placental membrane is called the "chorion", so this is where the 
"Dichorionic" comes from.  Diamniotic obviously refers to the two amnions.

3.  Monochorionic Diamniotic Monozygotic Twins - 1 placenta, 2 amniotic 
sacs.  Occurs when the ova splits at 4-8 days.  This is not as safe as 
#2 due to possible complications like "Twin-to-Twin Transfusion 
Syndrome", where the twins actually grow vessels between the two, 
causing issues due to asymmetric nourishment.  However, 95% of these 
twins make it just fine.  This is also by far the most common type of 
identical twinning.  There are two amniotic sacs but one placenta 
because the ova split before the amions differentiate but after the 
placenta does. 

4.  Monochorionic, Monoamniotic Monozygotic Twins - 1 placenta, 1 
amniotic sac.  Occurs when the ova splits at 8-13 days.  This is 
regarded as a very high-risk situation because the twins can get tangled 
up resulting in a loss of circulation.  I believe there are also other 
complications that can arise from this situation.  Modern anecdotal 
evidence says that 60% of these twins make it out fine, which is much 
much higher than the historical number of 20%.  There's only one each of 
amnion and placenta because the split occurs after both have already 
differentiated.

5.  Conjoined Twins - also known as "Siamese Twins".  Occurs when the 
ova splits after 13 days.  At this point the baby has begun to take 
shape and any division winds up causing a conjoined condition.

So with that background, the original question is still floating out 
there - "How do they know they're identical?"

If we look at the above, one would think that #1 and #2 would look the 
same (two babies, two placentas, two amniotic sacs), so it would be hard 
to tell.  On the other hand, #3 and #4 should be dead giveaways because 
you've got two babies and only one placenta (or in the case of #4, both 
placenta and amnion), right?  Wrong.  There's something called a "fused 
placenta", which could make a set of DIzygotic (fraternal) twins look a 
lot like #3 on the ultrasound. 

So how to tell for sure?

There are a few answers to this question.  One obvious, but not 
necessarily accurate, solution is to wait until they're born and see how 
much they resemble one another :)  This isn't nearly as good as most 
people realize though because a lot of features come about as a result 
of conditions in the womb - which can both make fraternal twins look 
alike (Mary Kate and Ashley, anyone?) or identicals appear a little 
different (they're never EXACTLY alike are they?).  Twins with different 
genders are also a giveaway, because it's not possible for identicals to 
be different gender, as their DNA is the same.

With that somewhat-unscientific solution aside, there are a couple 
others.  The solution that appears all over the websites is genetic 
testing.  For $125-175, you can do cheek swabs and send them off to be 
analyzed.  The results come back in 4-to-6 weeks, and voila you know 
what you've got.

So how do we know - even though the babies AREN'T born yet?  It's 
deceptively simple, should be very accurate from what I've read, but is 
rarely if ever mentioned the online literature.  In fact, it's hard to 
find reference to it even amongst discussions betwee people who've had 
twins, which makes me think that our doctor is really on top of things 
(as well he should be, seems he's got quite the reputation around town).

What our doctor did, was look on the ultrasound for the Corpus Luteum 
cyst that forms when the ovary releases an egg.  This cyst is perfectly 
normal, and performs several functions including the generation of 
hormones, etc.  So how to tell if they're identical?  If you have two 
cysts, it means she released two eggs.  If there's just one, then she 
only released one.  One egg == one zygote == monozygotic, or "identical" 
twins.  Seems that it's just that simple.

Now, there's one catch to all this - doctors suspect the existence of 
yet another twinning mechanism called "Polar Body Twinning", whereby the 
egg divides....BEFORE it's fertilized.  So what you get is a rather 
unique situation where the twins share *half* their DNA (the half 
donated by the egg).  Of course, counting the CL cysts would not be of 
benefit in this situation because there's still only one egg involved.  
However, it's thought that if the mechanism exists at all in humans, 
it's pretty rare. 

Anyway - there's a very long-winded answer to a fairly simple one-liner 
question.  If you've read this far, maybe you learned something....and 
hopefully I didn't screw anything up :)

As an aside, are any of the genetic hobbyists on the list capable of 
doing the genetic testing thing?  I think it'd be a cool thing for the 
baby book :)

-N
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