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What are the criteria for siblings to be twins

Discussion in 'Personal' started by mammal, Jul 10, 2011.

  1. It felt like that when I was pregnant with mine. A single pregnancy to follow wasa doddle in comparison!
  2. Different- seperate sacs - in the womb
  3. <u>Fraternal twins </u>occur when a mother ovulates twice in one month ie. releases 2 eggs, and both eggs are fertilised resulting in 2 embryos.
    This can happen at the same time or several hours, or maybe even 1 or 2 days apart.
    Fraternal twins can potentially even have different fathers, and this has happened.
    They do not sure a placenta, and obviously they don't share identical DNA. They can be different sexes.
    <u>Identical twins </u>occur when one egg is fertilised by one single sperm, and then divides into 2 embryos. They share a placenta and have identical DNA.
    It is however possible somehow for identical twins to be different sexes - don't ask me how, because I don't know for sure. It may be that the egg has divided before fertilisation and become fertilised with 2 different sperm, after it divided.

    I think it would be unusual for identical twins to be delivered 2 days apart, but I suppose it could happen in fraternal twins, as they don't share the membrane or placenta.
  4. I thought the criteria is that they have shared one womb?
    So identical twins are from one egg.
    Fraternal twins from two eggs.
    But in both cases, the womb is shared?

  5. Fraternal twins are not necessarily defined by the time/date of conception or the time/date of delivery, rather the fact that the pregnancies are concurrent.
  6. Fraternal twins are not necessarily defined by the time/date of conception or the time/date of delivery, rather the fact that the pregnancies are concurrent.

    It probably explains why they said that one of my twins was a premature baby and the other wasn't, even though they were born only 19 minutes apart.
  7. So, 2 pregnancies but at the same time/overlapping is enough for babies to be twins
  8. They are in the womb at the same time, are they not?
    I thought that was the definition?
  9. Separate womb sacs
  10. Hhhmm, I am not actually completely sure about the concurrent pregnancies, now that I think of it.
    I have heard it said that in IVF, where embryos are frozen and put back maybe years after, they can be described as twins.
    I shall have to go and investigate, lol!
  11. Maternal twins = twins ... no issue

    Perhaps Fraternal are simply siblings born at times that are close together
  12. The general consensus is (in my understanding) that twins are two individuals who share the same uterus and are usually, but not always, born at the same time.
    Fraternal - 2 separate eggs fertilised by 2 different sperm
    Identical - 1 egg which is fertilised and then divides into 2 embryos, and may or may not share amniotic fluid and placenta.
    There are some additional variations on the above.
  13. giraffe

    giraffe New commenter

    There's no such thing as maternal twins.
    Fraternal twins means they are not identical - separate eggs.
    Identical twins - same egg divides and so they share genetic make up.

    Twins are when a mother is pregnant with both at the same time, give or take a bit at the beginning or end.

    IVF messes around with this and can result in identical 'twins' being implanted separately in separate pregnancies, perhaps years apart.
  14. OK someone needs to explain this to me, if one is male and one is female then they are not identical, well physically at least.
    But the DNA? How does that work with the X and Y bits?

    I know that after a bone marrow transplant hte blood of the recipient has the DNA of the donor - does it work like that?
    Going to have me thinking for a while.

    Back to the origional post.

    One pregnancy, one labour, two children - yep I think they are twins.

    I think the sac thing is confusing the iddue. If you had three babies, all born on the same day, one pregnancy, one labour, but two babies shared a sac and one didn't would they be twins and a sibling or triplets?

    I think it is all about sharing a uterus.
    A man impregnantes two different women on the same night and they become pregnant - they are not twins.
    A woman is impregnated by two different men on the same night, or successive nights then I think the children will be twins.

    ROSIEGIRL Senior commenter


  16. I think............although I don't know for sure, depending on what time in the process the dividing of the fertilised egg occurs, you can get slight differences in the chromosomes, even though the DNA is otherwise identical - in identical twins.
    You could have identical twins, where one is born with Down's Syndrome, or some other kind of abnormality, and one is not, for example.
    Cases of boy/girl identical twins are very rare, and usually the girl will have the condition "Turner's Syndrome", as the Y chromosome is missing.
    I suppose I was being a little playful, when I said about the different sexes, but it is possible.
    Although identical twins share the same DNA, it will not always be identical. There can be small differences in it, depending on exactly how the genetic information divides. Apparently it is all to do with nuclear DNA and mitochondrail DNA - the differences occurring the mitochondrial DNA.
    Isn't google wonderful ;)
  17. OK more confused - I thought girls didn't have a Y chromasome. OK going to do some googling myself.

    ROSIEGIRL Senior commenter

    From what I've read, the boy/girl identical twin thing is extremely rare and the result of a chromosomal defect -
    'Among monozygotic twins. in extremely rare cases. twins have been born
    with opposite sexes (one male. one female). The probability of this is
    so vanishingly small (only 3 documented cases) that multiples having
    different genders is universally accepted as a sound basis for a
    clinical determination that in utero multiples are not monozygotic. When
    monozygotic twins are born with different genders it is because of
    chromosomal birth defects. In this case. although the twins did come
    from the same egg. it is incorrect to refer to them as genetically
    identical. since they have different karyotypes.'
  19. I did say it was rare and that I was being "playful" by introducing it.
  20. Sashh - they don't have a Y chromosome, which is why when the Y chromosome is accidentally dropped they egg divides, the embryo is female. The female twin only has the single x chromosome, and therefore will also have Turner Syndrome.
    The other theory is that, the egg divides before fertilisation, and is fertilised by a male AND female sperm. Although not strictly identical, they will both have come from the same egg.

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