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Medical Facts:
- Monozygotic ("identical") twins happen when one fertilized egg splits into two developing fetuses. If this happens during the first 2-3 days after fertilization, each baby will have its own placenta and membranes (diamniotic, dichorionic). If this split occurs on days 3-8, there will be separate amniotic membrane but will share an outer membrane called the chorion (diamniotic, monochorionic). If the split happens on days 8-13, the twins will be in the same amniotic sac (monochorionic, monoamniotic). When the split occurs on days 13-15, the twins will be conjoined. After day 15 of fertilization, twinning cannot occur.
 Monozygotic twins must be the same sex because they have the same genetic makeup. There is a one in four billion chance that after the egg splits, one fetus will lose a sex chromosome and this results in a normal XY male and a Turner’s syndrome female (XO). This has happened once this century and so is unlikely to happen again anytime soon.
- Dizygotic ("fraternal twins") occur when two eggs are fertilized by two sperm. These twins are not genetically identical and share no more genes than any siblings that were not born at the same time. All boy/girl twins are dizygotic (except as noted above), but they can be same sex as well.
- Usually twins can be seen to be either mono- or dizygotic by examining the placenta at birth. If any membranes are shared or the twins are in the same sac, they must be monozygotic. If the placentas of dizygotic twins implant close to one another they can appear to fuse, so if there is a dichorionic, diamniotic set of twins that are same sex, the only real way to tell is by DNA analysis (unless it’s fairly obvious that they don’t look a thing alike, although identical twins don’t have to be absolutely identical in appearance – generally they share the same hair and eye color).
- Twins births have risen 62% since 1980 (mostly due to interventional reproductive assistance and older age women having children)
- Identical twins can occur with IVF, eggs that are manipulated are more likely to form identical twins.
- 1 in 100 births is a twin.
- 1 in 250 births is an identical twin.
 Other facts:
- The term twin comes from the ancient German word "twine" meaning "two together".
- Twin speak or a language between twins that only they understand is known as "cryptophasia".
- In the US, more than 150 sets of identical twins are married to identical twins.
- There are 125 million living multiples across the world.
- The largest number of multiple births by one woman is 16 sets of twins, 7 sets of triplets, and 4 sets of quadruplets – 69 children born between 1725-1765 and 67 of which survived to adulthood.
- Chang and Eng Bunker (the original "Siamese" twins) were the oldest surviving conjoined twins. They lived to be 63 years old, married sisters, lived in NC, and fathered 21 children between them.
For more really fun facts about oldest twins, tallest twins and more go to:
www.twinstuff.com/twinfact1.htm
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