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Statistics on gender in fraternal twins?

post #1 of 18
Thread Starter 
My husband and I were debating this and I haven't been able to find any good statistics to back up either of us.

OK, so if you have fraternal twins, what are the chances of getting boy/girl twins? He says 50%, based on pure probability. But what about all the complex factors that go into gender selection? I mean, say you DTD 3 days before ovulation and happened to have a high dairy diet- studies have shown you're more likely to have a girl, and if so, wouldn't you then be more likely to have two girls if you're carrying twins? And vice versa for boys? If conditions in the mother's body, timing, and the dad's sperm count all contribute to what gender you get, it seems like you'd have a less than 50% chance of getting a mix, because conditions would favor one gender or the other.

Anecdotally, it seems like I know a lot more b/b and g/g twins than b/g.

Or is my husband right, and it's pure probability?

(I am just asking out of curiosity, because a friend of ours just found out she's having twins and we were chatting about it. )
post #2 of 18
I think that there would be a 33% chance for g/g twins, a 33% chance for b/b twins, and a 33% chance for b/g twins . . . thus a 66% chance for same-sex twins. But I was never very good at statistics, lol!

Lex
post #3 of 18
I think that for fraternal twins g/g is most common because there are more girls born than boys. Then b/g is 2nd and b/b third.

I'm not for sure, though. I'll go get my twins book in the basement at nap time and look it up because I remember reading it in there.

I have fraternal girls.
post #4 of 18
Quote:
Originally Posted by **Cat** View Post
I think that for fraternal twins g/g is most common because there are more girls born than boys. Then b/g is 2nd and b/b third.

I'm not for sure, though. I'll go get my twins book in the basement at nap time and look it up because I remember reading it in there.

I have fraternal girls.
I remember reading very recently that identical boys are the least common type of twin. I have to remember where I read that because I bet it has stats for other combos as well. I'll be back if I remember.

Oh but yeah, I think there are more singleton boys born that girls, actually, but boys have a higher mortality rate. So with twins I think it's more girls because more of the boys don't make it? I need to find where I read this.
post #5 of 18
Thread Starter 
Hm. It's so weird to me that there aren't better stats on this. You'd think there would be, somewhere!

My husband found a study about twin births in COWS that supported his argument, and tried to say that it counted.
post #6 of 18
Quote:
Originally Posted by lexbeach View Post
I think that there would be a 33% chance for g/g twins, a 33% chance for b/b twins, and a 33% chance for b/g twins . . . thus a 66% chance for same-sex twins. But I was never very good at statistics, lol!

Lex
I deleted my post before submitting this morning saying pretty much this same thing because I thought I might make a fool of myself if I was way off base Math is definitely not my strong point.
post #7 of 18
My husband is a math guy.... He assured me that with our twin pregnancy there HAD to be at least one girl in there!!!!!!!!! We have 5 boys!!!!!

Here are some stats from Wikipedia:

Types of twins

There are five common variations of twinning. The three most common variations are all dizygotic:

* male-female twins are the most common result, at about 40 percent of all twins born
* female DZ twins (sometimes called sororal twins)
* male DZ twins.

The other two variations are monozygotic twins:

* female MZ twins
* male MZ twins (least common).

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. There is also the mirror image variations: This is where the twins develop reverse asymmetric features. About 25% of monozygotic twins are mirror image twins. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,[9] while it is 1.07 males/female in Italy.[10] However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

Another variety of twins, "polar body twins," is a phenomenon that was hypothesized to occur and may recently have been proven, very rarely, to exist. This would occur when a portion of a mature egg separates itself from the egg. This is known as the first polar body, and it carries all the same genetic information as the egg. If polar body twins are fact, they would occur when two sperm fertilize both the egg and the first polar body. Generally the first polar body disintegrates. Polar body twinning would result in "half-identical" twins

http://en.wikipedia.org/wiki/Twin
post #8 of 18
Very interesting information. I have two fraternal boys!
post #9 of 18
Quote:
Originally Posted by DeannaK View Post
Very interesting information. I have two fraternal boys!

Yup, this is what I was going to say. I don't think I've even met another set of fraternal boys IRL. Most of the twins I meet/know are boy/girl. Not that personal experience makes up for statistics!
post #10 of 18
Quote:
Originally Posted by mama_tigress View Post
I remember reading very recently that identical boys are the least common type of twin. I have to remember where I read that because I bet it has stats for other combos as well. I'll be back if I remember.

Oh but yeah, I think there are more singleton boys born that girls, actually, but boys have a higher mortality rate. So with twins I think it's more girls because more of the boys don't make it? I need to find where I read this.
I read that somewhere as well ... and we have ID twin boys!! I have a GF who has ID twin girls.
post #11 of 18
Quote:
Originally Posted by Katielady View Post
If conditions in the mother's body, timing, and the dad's sperm count all contribute to what gender you get, it seems like you'd have a less than 50% chance of getting a mix, because conditions would favor one gender or the other.
If the dad's sperm are primarily more X's than Y's (or vice versa) that would have an effect for you individually, but presumably not at a broader level of trends for twins. Also, the degree to which the timing and mother's diet and all of that has a factor in gender of the babies would probably be skewed in the statisics for twins due to the percentage of twins born from fertility treatments, in which those factors would seem to be less at issue. I'm just totally guessing, though. Very interesting question!
post #12 of 18
Okay, I looked it up in Elizabeth's Noble's, "Having Twins," because I remembered reading about this somewhere. It says:

"Approximately half of dizygotic twins are same-sex pairs and half are male-female (although in Holland and Sweden same-sex dizygotic twins occur 10 to 15 percent more often than male-female pairs)."

But because 1/3 of all spontaneous twins are monozygotic (and thus, same sex), about 1/3 of all twins (mz and dz combined) are b/g, 1/3 are b/b and 1/3 are gg.

Lex
post #13 of 18
Ugh! too much science for me , I have frat twin girls but was told that it was because of my age and I released 2 eggs instead of one. We were researching some fertility issues online and the thing that stuck out for me was the high percentage of my having had a second set of twins, again due to age and having already had a set of twins. I've had a tubal but am considering a reversal.
post #14 of 18
I read somewhere, and I'm looking for the info to back this up, that there's a 50% chance of b/g twins and a 25% chance of each single gender fraternal twins.
post #15 of 18
Quote:
Originally Posted by lexbeach View Post
Okay, I looked it up in Elizabeth's Noble's, "Having Twins," because I remembered reading about this somewhere. It says:

"Approximately half of dizygotic twins are same-sex pairs and half are male-female (although in Holland and Sweden same-sex dizygotic twins occur 10 to 15 percent more often than male-female pairs)."

But because 1/3 of all spontaneous twins are monozygotic (and thus, same sex), about 1/3 of all twins (mz and dz combined) are b/g, 1/3 are b/b and 1/3 are gg.

Lex
This is what I had read as well.

Ours are boy-girl!
post #16 of 18
I like those odds-- as I'm really hoping for b/g! (Although I'll of course be thrilled, thrilled with any combination-- but it was almost impossible for me to get pregnant, so I think this is our only shot, so one of each would be cool. But I'm just so lucky to be having any!)
post #17 of 18
Quote:
Originally Posted by mamamolly1 View Post
but it was almost impossible for me to get pregnant, so I think this is our only shot, so one of each would be cool. But I'm just so lucky to be having any!)
I know how you feel! :
post #18 of 18
I wasn't even looking for this, but I stumbled upon it when looking for growth stats today. Figured it was timely post here. For what it is worth.
Quote:
Multiple Facts.

The natural incidence of twins is one in eighty-nine births.
About two thirds of all twins are fraternal. Half of these pairs are boy/girl; one fourth are girl/girl; and one fourth are boy/boy.
It was on this site.
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