When is a twin, a twin?

The comment on yesterdays post set me thinking again about, what is a twin?  When we see a twin pair walking about, what are we looking at?

IDENTICAL ( monozygotic) twins

• They may be two survivors of a monozygotic triplet or even a quadruplet set.
• There could have been a third, non-identical embryo that was there for just a little while. That is common in IVF where more than one embryo is transferred. Monozygotic twins are more often found in IVF pregnancies than naturally conceived pregnancies.

FRATERNAL ( dizygotic twins)

• There may have been an extra non-identical embryo  (or more than one) that only lived for a little while
• One of the twins may be the sole survivor of a monozygotic pair
• Each twin may be the survivor of a different monozygotic pair (originally a quadruplet pregnancy)

A twin, surely is someone who was conceived as a twin and that is all that happened - there were no other embryos that were there  for a while. If there  was another embryo, perhaps more than one, these twins are both womb triplet survivors.

We now know that the loss of a  womb mate before birth has a profound physical and psychological effect on the survivor and that effect varies with zygosity.  That means that fraternal twins may include a monozygotic womb twin survivor,  and it is known that monozygotic womb twin survivors are at particular risk of such things as cerebral palsy and developmental abnormalities such as split organs.  This is not mention the psychological effects, which the WombTwin research project has only just begun to explore.  (More here)

Twin research is at risk of getting false results if they dont take this important factor into account.