1. twins
- defined as those born at the same time or of the same pregnancy.
- may be fraternal
identical or
conjoined
Source:

2. Identical twins begin as a single fertilized egg (zygote),
MONOZYGOTIC TWINS
Identical twins begin as a single fertilized egg (zygote),
THEN…

3. the split

4. 2 separate, but identical, individuals.
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

5. Timing is Everything!
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6. Dichorionic diamniotic
If 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.

7. Dichorionic diamniotic
These 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.
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8. Monochorionic diamniotic
If, 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.
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9. Summary of Two Types
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10. 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.

11. Mo-Mo’s
Mo-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.

12. Mo-Mo’s
Mo-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

13. Disk and Risk
On 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:

14. Conjoined twins
Conjoined, 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.

15. “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.

16. Conjoined facts to ponder
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

17. Types of Conjoined Twins
Diagram sources:

18. Attached at back of the head
Craniopagus:
Attached at back of the head  
Separation is possible although brain damage is common
Diagram sources: ,
A website dedicated to support, education and research regarding conjoined twins

19. Types of conjoined twins
Rachipagus:
Very rare – union at back of spine
Diagram source:
A website dedicated to support, education and research regarding conjoined twins

20. Separation is possible, but is indicative of prosthetic limb/s
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
Diagram source: , A website dedicated to support, education and research regarding conjoined twins

21. These twins do not survive. Ischiopagus:
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: A website dedicated to support, education and research regarding conjoined twins

22. Omphalopagus: Thoracopagus:
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: ,
A website dedicated to support, education and research regarding conjoined twins

23. Parasitic
Additional limbs, torsos or heads, believed to be caused by the death of one twin in utero.
Diagram source: A website dedicated to support, education and research regarding conjoined twins

24. Why does twinning happen?

25. 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

26. Twin births
Common 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

27. sites to visit
references