Information on Identical Twins
Most information on identical twins will tell you that they account for 25 to 33% of all twins.
Identical twins are also called monozygotic twins.
They result when one egg splits in two.
Information on Identical Twins:
Different Types of Identical Twins
In about 35% of cases of identical twins, the egg splits within the first 3 days after conception.
In this case, the babies will have separate placentas.
They are called Dichorionic - two placentas and Diamniotic - two amnions. The amnion is the sac that surrounds your unborn baby.
However, in about 65% of cases, the splitting occurs at the end of the first week after conception.
This means the babies will share a placenta.
They are called Monochorionic - one placenta and Diamniotic - two amnions.
1 to 2 % of identical twins share a placenta and one amnion - bag of water.
They are called Monochorionic - one placenta and
Monoamniotic - one amnion.
This situation can be a very dangerous as the babies' umbilical cords often become entangled.
This can lead to the babies' death.
If the egg division occurs at 12 to 13 days after fertilization then the twins will share one amniotic sac - bag of water - and will share one placenta.
However, they will also share body parts.
There are called conjoined twins and this occurs 1 in 40,000 total births - 0.5 % of monozygotic twins.
More Information on Identical Twins
Fetal to fetal - baby to baby - connection of the blood vessels in the placentas is called
anastomosis.
It occurs to some degree in all identical twin pregnancies.
Anastomosis can occur:
Artery to artery
Vein to vein
Artery to vein
An artery-vein connection can lead to a potentially dangerous complication called Twin-to-Twin Transfusion Syndrome also called TTTS.
Twin-to-Twin Transfusion Syndrome - TTTS
TTTS affects identical twins who share the same placenta.
The shared placenta contains blood vessels which connect the blood circulation of the twins.
Often the common placenta is shared unequally by the twins. This means one of the babies may not have
enough of the necessary nutrients to grow at a normal rate and/or survive.
TTTS is a random event. It is not hereditary nor is it genetic, neither is it caused by anything
you - the parent - did or did not do. Sadly, TTTS can happen to anyone.
Depending of the extent of the shared vessels connecting the babies, the blood supplies can be transfused
disproportionately from one baby called the donor to its twin called the recipient.
The transfusion causes the donor to have decreased blood volume.
This leads to:
Slower than normal growth
Poor urinary output resulting little to no amniotic fluid. Did you know that most of the amniotic fluid is urine from the baby?
The recipient twin becomes overloaded with blood.
This excess blood puts a strain on this baby's heart to the point that this baby:
May develop heart failure
May have too much amniotic fluid
TTTS can occur at any time during pregnancy, even while a mother is in labor at term.
Chronic TTTS describes those cases that appear between 12 to 26 weeks.
This is very dangerous since these babies are too immature to survive outside the womb even if delivered.
The mortality rate for these babies - sadly - is over 80%.
Acute TTTS describes those cases that occur suddenly.
Remember: Most identical twin pregnancies are without problems.
Here is some recent information on identical twins and TTTS:
A five year identical twin study conducted by Drs. Y. C. Senga and V. S. Rajadurai showed an incidence of TTTS for identical twins at only 6.2%.
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