Identical or Fraternal?

[Adaline'sMama]

Can we clear this confusion up once and for all, because Im still confused about it......

What are the chances of di/di same sex twins being identical? I know in a couple of other threads we have gone over it, and done some math, but I didnt really get it and now I cant find it. Can someone (Adorkable, I think) tell me again?

So, we are having di/di girls (Im 31 weeks pregnant). I feel fairly sure that they are fraternal, mainly because I believe I ovulated twice (OPK strips were positive about 48 hours apart with a negative in between, anyway). But, if they look alike at birth, then we will likely get a dna test because DH really wants to know and the testing is pretty cheap. 

[Emelee]

My Dr.said if I ovulated 2x then only 25% chance would both be girls or otherwise it was around 35% chance they were identical since I had di-di girls. He didn't explain why the 35% chance and I didn't think to ask. My girls are fraternal, I could tell right away because their facial structures are so different. Why not just get the test done if it's so cheap? If they had looked too similar I would have done it just to be sure.

[13pumpkins]

We have both, so to speak.  I am not sure about the exact statistics but I do know that Emelee's OB is correct about a 25% chance of having same sex fraternals.  That is the same as the statistics between successive singletons.

We planned on having the babies tested as it was too much debate over their zygosity and as Emelee pointed out the test is readily done.  The first U/S where they were discovered really made it look like B and C were di/mono identicals.  The next U/S did as well but on the third one, the peri insisted they were not.  Then weeks later he, the peri, said the chances of us spontaneously releasing three eggs was low so it was probably two were identicals.  Back and forth.  In the end, our trusted OB was correct as it was found upon delivery that there was two aminions and one chorion for babies B and C.  LOL.  Test or no, it is really obvious.  B and C are carbon copies physically and A is very different.

[studentDr]

are you on ovulation hormones ?

what I have been taught might be visible on the ultrasound , depending n the resolution I guess

I think it goes like this

1. there is di zygotic meaning two eggs and two sperms. then there has to be two placentas (dichironic) and two amniotic sacs (diamniotic) and the babies will not be identical because ther are two different eggs. because ther are two different eggs there is the same chanceofhaving either a boy or a girl as withany normal pregnancy . ie 50% chance that one will be a boy or a girl. so yes a 25% chance that they will both be of the same sex looking at the possible combinations

girl girl 25%
girl boy 25%
boy girl 25%
boy boy 25%

I had some friends when I was a kid who were di zygotous girls and it was very easy to tel them apart.

. . .

2. or monozygotic meaning that there is one egg and sperm that forms a cell mass (zygote) that splits

so if they are monozygotic twins then they have to identical

the number of sacs and placentas in the monozygotic twins depends on the stage that the egg cell clump splits at .

a) split Before day 3, the placentas are fused, but there wil be two placentas and two sacs (dichironic diamniotic)

b) split between day 3 and day 8 ( the blastocyst implants and the inner mass divides) so there is one placenta and two sacs , (monochorionic, diamniotic)

c) split between day 8 and day 13 results in one placenta and one sac, (mono chorionic, minoamniotic)


* reference: king Edward memorial hospital postgraduate training website 2012

so if you definately see just one placenta then they are identical. and if the placentas are fused, most likely identical.

if the placentas are located in totaly different parts of the uterus then probably nonidentical.



. . .

and I would expect that the chance of having an egg split (identical twins ) is less than the chance of having two eggs released which would be high if you are on ovulation hormones (non identical twins. )


. . I'm guessing that it might not be that easy to see if the placentas are fused or not in such a small space, I will leave that comment up to more experienced people on here
Edited by studentDr - 12/9/12 at 9:39am

[13pumpkins]

The wild thing for me is how many people asked us all of these questions.  What happened for us is two eggs were released and fertilized and then one conception split again.

My favorite answer was our OB's when DH asked him if there were twins in there (meaning if we had a pair of ID twins in there)...he said, "it's not twins, it's triplets!"

While we ended up with what is jokingly called a pair and a spare we really are trying to demphasize the whole twin concept for us because I think the ramifications of one baby growing up as the "spare" is horrible.  I am not sure how it is with having twins but we do get a lot of attention for three and lots of fuss over the two that are identical.  I like my OB's take on it because really past answering people's question, it is of low significance.  We are glad to know in the case of genetic disorders but even then, there are studies that show that while ID twins have the same genetics, they can have different expression due to environmental influence.

[Adaline'sMama]

Just to clarify: They are both  girls, they are for sure di-di (the placentas are fused, but a level two us confirmed di-di), and they were spontaneously conceived. No hormones or anything. I did get pregnant in a very, very stressful time in my life and on the first cycle after I very abruptly stopped nursing. Also, we will get the test done unless one of them has blonde hair and one has black or something like that. 

So, am I seeing correctly that the chances of di-di same sex twins being identical is 35%  ?

[studentDr]

if the placentas are fused then they are monozygous, 100% chance of them being identical

the day at which the one zygote splits might change the number of placentas or sacs but it is still just the one zygote. one zygote means the exact same cell bunch created from the one egg and one sperm fusion

. . .

usually if they're not identical one has a distinct birthmark or something

. . .
Edited by studentDr - 12/8/12 at 9:33am

[studentDr]

I've got some good information on antenatal management of twin pregnancies if you would like me to post it

[studentDr]

oh I just read that of all twins 2/3 are dizygotic and 1/3 are monozygotic

twins are more likely with increased maternal age, height and weight
chance of twins by race is 1/30 African, 1/80 Caucasian 1/1000 Asian
family history
and of course fertility treatment

[Adaline'sMama]

Hmm...my OB, who also has a set of her own twins, says that often times fraternal twins have fused placentas, but they are still two separate placentas, acting completely separately. I dont think that them having fused placentas means that they are 100% identical.

I am 28, have had two other children, am 5 foot even, slightly overweight (carrying baby weight, this will be 4 babies in 3 years), and have had no fertility treatments and with no history of twins in our family. 

How exactly is antenatal care any different than prenatal care? 

[eminvc]

In Dr. Luke's book, I think she says that of twins that are di/di, 25% are identical and 75% are fraternal...I've been telling people then that I have a 25% chance of having identical twins. Not sure if that's correctly calculated, but I find most people (including current twin parents) are totally confused and so convinced that 2 placentas=fraternal twins, this math suffices.

For my own curiosity, though, I would love to know in advance! I feel that mine are going to be fraternal due to the fact that my father is a fraternal twin w/ his brother so I think it's genetic...but we will see! Yet one more surprise to go.
 

[Emelee]

I'm pretty sure that even if the placentas fuse it does not mean 100% identical. It depends on where the eggs implant and how the placentas grow. There was someone who said their twins' placentas ended up fusing and it was boy/girl so no chance of identical. Also, they can still be identical and not have fused placentas. The Obs and ultrasound techs made sure to remind me of this often because of an anterior and posterior placentas I assumed fraternal (which they ended up being) but they said there was still a chance.

Also, eminvc, I think that the fraternal twin genetics come from mother's side of the family. So if you mom/mom's sisters/your own sisters have fraternal twins you have a greater likely hood. Correct me if I'm wrong, that's just what I remember reading.

ETA: I've done a quick google search and it's saying anywhere from 4% to 33% (depending on who you ask) is the possibility of your di-di girls being identical. That's a lot of help 

[studentDr]

probably dizygotic placentas could fuse.

but if they are fused from the start. which you probably couldn't see on ultrasound at such an early stage then that's the first mononzygous split
I'm just quoting theory. I don't have any experience in how hard it is to tell by ultrasound, it's probably very difficult

[~Adorkable~]

.

3.75%

Edited by ~Adorkable~ - 12/8/12 at 9:39pm

[~Adorkable~]

we did the math a while ago, based on a lot of states and taken from states that were out before the common use of fertility drugs messed the numbers up big time.  i'll look for the post

though taking fertility drugs does not mean your twins will always be fraternal, i know a mom in my twin club that did IVF and has very very identical girls, they got the m tested because everyone that knew they did IVF kept saying that ment they were DZ, nope

and a "fused" placenta means only that they are di/di and the two join together, it has no bearing on  the zigosity of the twins at all.

[~Adorkable~]

as a side note i met a lady a few weeks ago that has two set of twins, her oldest were 14 and younger ones are 3 or 4. anyway she at first told me that both were fraternal. but after we talked for a while i got that she had been told that solely on the fact that they were di/di. and while her younger boys were clearly fraternal, her older girls i could not tell them apart for anything! apparently neither could she for nearly 8 years!  i talked to her about the difference of di/di vs DZ and how yes her twins could in fact be MZ (one of the common sense rules are if mom can't tell them apart after a short while they are often MZ)

she is actually getting her 14 year old twins testing now, how crazy if they had been told wrong all this time, i think they have

on a funnier note i yet again got asked in my B/G twins were identical! and once again gave my favorite response, "nope, that one has a penis"

[~Adorkable~]

Originally Posted by Emelee otherwise it was around 35% chance they were identical since I had di-di girls. He didn't explain why the 35% chance and I didn't think to ask.

Originally Posted by Adaline'sMama 
So, am I seeing correctly that the chances of di-di same sex twins being identical is 35%  ?

sadly this number get thrown around a lot of it is used backwards. its not that di/di twins of the same sex havea 35% chance of being MZ, its that MZ twins have a 35% chance of being di/di. Statistics area tricky game and dont go in both directions.

Originally Posted by studentDr 

if the placentas are fused then they are monozygous, 100% chance of them being identical

the day at which the one zygote splits might change the number of placentas or sacs but it is still just the one zygote. one zygote means the exact same cell bunch created from the one egg and one sperm fusion

. . .

usually if they're not identical one has a distinct birthmark or something

. . .

studentDr, sadly a lot of what you are saying is either wrong or out of context. 

we love to have you and I applaud you getting your medical training that i assume is referred to by your name.  please stick around anywhere on mothering.com, we all learn a lot from each other.

Originally Posted by studentDr 

I've got some good information on antenatal management of twin pregnancies if you would like me to post it

and please do not offer medical advice on the board, its not the right place for that, folks should get true medical advice from the professionals in their real life that know their situations.

[~Adorkable~]

Originally Posted by studentDr 

probably dizygotic placentas could fuse.

but if they are fused from the start. which you probably couldn't see on ultrasound at such an early stage then that's the first mononzygous split
I'm just quoting theory. I don't have any experience in how hard it is to tell by ultrasound, it's probably very difficult

"fused" by nature of the word means they were separate at one point, it is medically different than "one" it cant be fused trull from the start, it was either one or two and then grew from there .

Mo/Mo MZ twins and many maybe most Mo/Di MZ twins have ONE placenta. Tons of Di/Di twins both MZ & DZ have fused placentas, it means very little, its like two tree growing really close to each other and finally squishing into each other, they are still two trees and each feeds itself.

[~Adorkable~]

ok here s the math i did in a post a long time ago when they cam up before. if my math is wrong in anyway i'm happy to be shown

ok here is some math nerd for y'all.

I'm using the numbers roughly from the mid 80's since that is the only way to take fertility treatments out of the numbers.

 if no fertility treatment was used and all other things are equal...

• chances of giving birth to twins about ~20 per 1000 back then or 2%
• chances of giving birth to MZ twins ~3 per 1000 or .3%
• number of MZ that are Di/Di  ~25% of the .3% mentioned above or .075%

So...

.075% is 3.75% of 2%  (happy for someone to check that math, please)

so that doc was not that far off if he didn't take into account that the twins were Di/di, since those numbers would have been ~15%, if no fertility treatment was used and all other things are equal

but since he saw them as di/di, the number get way way smaller. With there only being a 3.75% chance of them being MZ

am i thinking this out correctly?

[Emelee]

Great info Adorkable! Maybe what my OB meant was since there's about a 1/3 chance twins are identical then he estimated/rounded up to 35% instead of 33.3333333%. That means he was not taking into account that they were di-di only that the chance was 1/3 they could be identical. So that means it would be a much smaller percentage of di-di same gender would be identical. I'm not super hot with statistics but it's probably a smaller percent like you were saying.