- defined as those born at the same time or of the same pregnancy.

- may be fraternal

identical or

conjoined

Source: http://www.lemenille.com/twins.jpg

IDENTICAL TWINSMONOZYGOTIC TWINS

Identical twins begin as a single fertilized egg (zygote),

THEN…

that creates

2 separate, but identical, individuals.

IDENTICAL because they have the same DNA

Therefore, they have the same

* gender

* blood type

* hair & eye color

* genetic disorders

Timing is Everything!

Source: http://www.centrus.com.br/DiplomaFMF/SeriesFMF/11-14weeks/images-thefetus/gem-04.jpg

Dichorionic diamnioticIf the split occurs before cells begin to

differentiate at 4 days, the monozygotic twins will implant as 2 separate blastocysts. Each will have his/her own chorion and therefore his/her own amnion, as well.

These twins are then called dichorionic (2

separate chorions) diamniotic (2 amniotic sacs).

(NOTE that in this case, they are traveling through the oviduct when they separate.)

This occurs in 1 of every 4 twin sets.

Dichorionic diamnioticThese twins will develop in

their own amniotic sac, each with his/her own placenta. This separation decreases the risk of entanglement and twin-twin syndrome (described below) that occurs with greater frequency in monochorionic twinning.

Source:http://bidmc.harvard.edu/content/bidmc/departments/radiology/files/fetalatlas/gestations/dimono/xyz_b9737_19b.jpg

Monochorionic diamnioticIf, however, they separate between days 4-8, the

twins will share a chorion/placenta but have separate amniotic sacs.

Thus, they are referred to as monochorionic

diamniotic twins.

3 in 4 sets will develop in this way.

Source: http://bidmc.harvard.edu/content/bidmc/departments/radiology/files/fetalatlas/gestations/dimono/xyz_s9897

Summary of Two Types

Source: http://www.centrus.com.br/DiplomaFMF/SeriesFMF/11-14weeks/images-thefetus/gem-04.jpg

Late and Rare

If they split between the 8th and 13th day, then twins will share the same chorion AND amnion – monochorionic monoamniotic

(Mo-Mo’s)

(Keep in mind: they were implanted in the endometrium as one, THEN split.)

Only 1-2% of monozygotic twins occur this way.

Mo-Mo’sMo-Mo’s will share the placenta, as well as

the amniotic sac. That means there is a risk of being entangled/strangled by the umbilical cord/s.

The fetal heart rate is often tested daily to check for entanglement

This risk decreases as the twins mature, there is less room and less movement.

Mo-Mo’sMo-Mo’s share a placenta, therefore, blood

vessels often go between the two. This can lead to imbalance of blood flow, called twin-twin syndrome (TTS). That leaves one twin getting less blood and produces less urine. This twin is often much smaller than the other. Extra blood flow to the other, however, may result in heart failure.

Untreated, TTS may terminate the pregnancy

Disk and RiskOn day 13, the embryonic disk (bilayer of

epiblast and endoderm) begins to differentiate.

If the split occurs after day 13, the twins will share a chorion and amnion. They will also share body parts. They will be conjoined (Siamese) twins

Only monozygotic twins can be conjoined.

Source: http://home.att.net/~drdipstik/pow/pow041.jpg

Conjoined twinsConjoined, often referred to as “Siamese”

twins were once thought to be the stuff of myths and legends. With modern medicine and global communication, we are well aware that this incomplete division of identical twins really does occur. Television shows have documented the drama of having conjoined twins separated through very complex and surgeries.

“Siamese”

Conjoined, or “Siamese”, twins are so named due to the common recognition of the set of male conjoined twins of Siam. Born in 1811, they acquired ‘celebrity status’ through public exhibition in the U.S. and England.

Though remained attached by skin and liver and married (sisters) and conceived a combined total of 21 children.

Conjoined twins are 3 times more likely to be female than male

1 live set of conjoined twins is born in every 200,000 births

Country of origin influences frequency

Conjoined facts to ponder

Types of Conjoined Twins

Diagram sources: http://i.cnn.net/cnn/2003/WORLD/asiapcf/south/08/18/india.twins/vstory.india.twins.afp.jpg

Craniopagus:

Attached at back of the head  

Separation is possible although brain damage is common

Diagram sources: http://www.conjoinedtwinsint.com/types.htm , http://www.hopkinschildrens.org/images/news/craniopagus_twins.jpg

A website dedicated to support, education and research regarding conjoined twins

Types of conjoined twins

Rachipagus:Very rare – union at

back of spine

Diagram source: http://www.conjoinedtwinsint.com/types.htm A website dedicated to support, education and research regarding conjoined twins

Diagram source: http://www.conjoinedtwinsint.com/types.htm , http://www.conjoined-twins.i-p.com/twins.jpghttp://www.conjoined-twins.i-p.com/bacjoin.jpg A website dedicated to support, education and research regarding conjoined twins

Parapagus:

Side union, limbs vary

Separation is possible, but is indicative of prosthetic limb/s 

Pygopagus:

Union at back of pelvis

Low frequency of occurrence, but survival rate is high

Cephalopagus:

Union is in the front at the chest and head with 2 faces on opposite sides

These twins do not survive.

Ischiopagus:

Union is at the front of the pelvis - many shared internal organs.

Separation survival rate is good - excretion and sexual functions may be compromised.

Diagram source: http://www.conjoinedtwinsint.com/types.htm A website dedicated to support, education and research regarding conjoined twins

Omphalopagus:Union at front of abdomenal region – liver tissue may be

shared.

Separation of these twins has the highest rate of survival.

Thoracopagus:Union is at the front of the chest – the heart may be

shared.

These twins do not survive separation

Diagram source: http://www.conjoinedtwinsint.com/types.htm , http://cellar.org/pictures/moretwins.jpgA website dedicated to support, education and research regarding conjoined twins

Parasitic

Additional limbs, torsos or heads, believed to be caused by the death of one twin in utero.

Diagram source: http://www.conjoinedtwinsint.com/types.htm A website dedicated to support, education and research regarding conjoined twins

Why does twinning happen?

?

Don’t know

about 30% of twins are identical yet the causes

of monozygotic twinning are not known

monozygotic twinning is NOT genetic, BUT

the frequency of monozygotic twinning does increase with mother’s age, especially over 35

Twin birthsCommon complications with twin births exist for

mom and baby and include:

* higher risk of gestational diabetes * preeclampsia (high blood pressure, protein in urine,

swelling) * pre-term delivery (due to rupture of membranes) * undersized babies

* risk of breech delivery

sites to visit• http://zygote.swarthmore.edu/cleave4b.html• http://www.uhrad.com/pedsarc/peds034.htm

references• http://www.vh.org/adult/provider/obgyn/twinpregnancy/• http://www.dhmc.org/monochorionic_twins.cfm