08 October 2009

Twin Zygosity

The question I'm asked most often right now with the girls is: Are they identical? My answer is an honest "I don't know."

If twins are the same gender there a few ways to tell if they are identical outside of DNA testing. Most of these signs are linked with the day the zygote split in two after fertilization.

If the egg split between days...
  1. 1-3 The identical twins will have two separate chorionic and amniotic sacs and two separate placentas. Short of DNA testing, the parents would have no indication that the twins were identical.
  2. 4-8 The identical twins will have two separate amniotic sacs and two separate placentas, but share a chorionic sac (the outer membrane around the two). This is a rather hard situation to detect via ultrasound, so these parents may have no indication either.
  3. 8-13 The identical twins will share chorionic and amniotic sacs and a placenta. These twins are referred to as Mono/Mono and are usually detected by ultrasound and monitored very carefully for twin to twin transfusion syndrome. Their parents know that the twins are identical in the womb.
  4. 13-15 The identical twins will be born conjoined. In this case, you know they are identical.
What complicates matters further is that in cases 1 and 2 the placentas can fuse together and look like one placenta at delivery. Unfortunately, the fused placenta can occur in dizygotic/fraternal twins as well.

My girls were delivered with one gigantic placenta and look a lot alike. So your guess is as good as mine. If you're dying to know the answer to the girl's zygosity, I'll gladly let you fund a DNA test that'll put you out about $100 (or more). If the girls want to know... well, they had better start saving their allowances...


So in short, your guess is as good as mine and I'd love to hear it in the comment section ;)

7 comments:

Deanna said...

They sure do look a lot alike in that picture...but goodness, who knows??

Great explanation of twins and zygosity (great word, too!). My girls were monochorionic/diamniotic, so we knew all along they were identical.

Michelle said...

I just found out recently that boy/girl twins can be identical. I am not sure how and have not researched it further, but it is obviously extremely extremely rare. I have heard this from two different people. So I guess, when they ask my if my boy/girl twins are identical, I can no longer look at them like they are idiots. ;)

Melissa Ann said...

Boy/girl twins can be identical in very rare cases. Your boy would be a normal XY chromosome pair, but your girl would have Turners Syndrome. She would only have one X chromosome and would be missing the other chromosome of that pair. It happens rarely that the egg splits and for some reason the Y chromosome does not get transfers. You would know by now if your daughter had Turner's Syndrome as these girls are extremely petite and have other physical abnormalities.

Melissa Ann said...

So, yes, you can still look at them like idiots ;)

Chelsea P said...

Thank you for posting a link to your blog on Facebook. :o) I love to keep up! They are so cute, Melissa. I vote identical. I don't know, but that's my guess.

Derek Aldrich said...

I still say they are identical

Melissa G. said...

This was so interesting! I didn't know all that about twins!
Your girls are precious!