1. TWINNING

2. TYPES OF TWINS FRATERNAL TWINS IDENTICAL TWINS Two different eggs
Same egg
Two different sperm
Same sperm

3. Identical/Fraternal

4. Identical Twins

5. Identical Twins - cont. 3 in 1,000 births
No discovered genetic predisposition identical twinning
Older mothers have a higher chance of having twins
When the egg hatches out of the “shell” this can cause the blastocyst to split
The older the egg, the harder the “shell”

6. Timing of Identical Twinning
The timing of the splitting of the embryo can affect the number of placentas and the number of amniotic sacs
Splitting days twins are the “most identical”
often have two placentas - sometimes this leads to doctors thinking they are fraternal twins
Splitting days most common
often one placenta and two amniotic sacs
can result in “mirror image” twins
Splitting days 13-15
greater likelihood of conjoined twins

7. Differences in identical twins
Epigenetics

8. Interesting Identical Twin Cases
Identical twins marrying identical twins, having identical twins

9. Interesting Identical Twinning Casehyperlink
Two sets of male twins in Columbia switched at birth discover each other in their early 20s

10. Fraternal Twins 1 in 90 births
There is a genetic predisposition for fraternal twinning
The older the woman, the greater the chance of having fraternal twins
Thought the the response time for communication between the ovaries and pituitary gland is slower - this can lead to higher levels of FSH = more follicles can be stimulated
Can be different genders
As similar (genetically) as any two siblings

11. Unique Twinning 1. Conjoined Twins
Conjoined twins are monozygotic multiples that do not fully separate from each other due to the incomplete division of the fertilized ovum. The individuals will be connected at certain points of the body, and may share tissue, organs or limbs.
2.  Twins Conceived Separately: Superfetation
Normally when an egg is fertilized, a woman's cycle is interrupted and ovulation ceases. Rarely, however, an egg can be released while a woman is already pregnant, resulting in twins that are conceived at different times. 
3.  Twins with Different Fathers: Heteropaternal Superfecundation
Fraternal (dizygotic) twins are the result of hyperovulation, the release of multiple eggs in a single cycle. Superfecundation describes a situation where the eggs are fertilized by sperm from separate incidences of sexual intercourse. In a case where a woman has sex with different partners, the twins could have different fathers and the appropriate term is heteropaternal superfecundation.
4.  "Half Identical" Twins: Polar Body Twins
There are two types of twins, right? Dizygotic (fraternal) twins result when two eggs are fertilized. Monozygotic (identical) twins come from a single fertilized egg that splits. But what if the egg splits and then each half meets a sperm? That's the proposed theory for polar body or "half- identical" twins, twins who are very much alike but aren't a 100% DNA match. Although it seems to be a reasonable theory, there is no definitive test to confirm polar body twinning.
5.  Boy/Girl Identical (Monozygotic) Twins
Identical (monozygotic) twins are always same gender because they form from a single zygote that contains either male (XY) or female (XX) sex chromosome. However, there have been a few reported cases of a genetic mutation in male twins where one twin loses an Y chromosome and develops as a female. The female twin would be afflicted with Turner's Syndrome, characterized by short stature and lack of ovarian development. Of course, another explanation for gender differences in identical twins is an identical twin who undergoes a sex change operation.
6.  Mirror Image Twins
Mirror image twins are monozygotic, twins that form from a single fertilized egg. When the split occurs late - more than a week after conception - the twins can develop reverse asymmetric features. This term is not really a type of twin, just an way to describe their physical features. For example, they may be right- and left-handed, have birthmarks on opposite sides of their body, or have hair whorls that swirl in opposite directions. In theory, if the twins faced each other, they would appear to be exact reflections of each other. About 25% of identical twins are mirror image twins. More »
7.  Parasitic Twins
A type of conjoined twins that develops asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin. Manar Maged gained notoriety after being featured on Oprah. A variation of parasitic twinning is fetus in fetu, where an abnormally formed mass of cells grows inside the body of its monozygotic twin. It survives during pregnancy, and even occasionally after birth, by tapping directly into the blood supply of the host twin. This report describes an Indian man whose fetus in fetu was discovered as an adult. More »
8.  Semi-Identical Twins
A type of twinning identified in a pair of three-year-old twins in Described as identical on the mother's side but sharing only half their father's genes, the rare twins developed when two sperm fertilized a single egg, which then split. One twin is a hermaphrodite being raised as a female, with both testicular and ovarian structures, while the other is anatomically male. More »
9.  Twins with Different Birthdays
The most common explanation is a labor and delivery that begins before midnight on one day and ends after the clock changes to the next day. If that day happens at the end of the month, or even on New Year's Eve/Day, the two babies can have birthdays in different months and even different years! Also, sometimes a pregnancy is prolonged to provide each baby with an optimal chance for survival. If preterm labor forces the delivery of one baby, doctors can successfully control the abor and delay delivery to give the other baby more time in the womb. Twins and higher multiples have been born days and even weeks apart. More »
10.  Twins of Different Races
Heteropaternal superfecundation can explain cases of fraternal (dizygotic) twins with differing racial characteristics. In one case, the differentiation was due to a lab mix-up during an in-vitro procedure. However, in in the United Kingdom, two bi-racial parents conceived fraternal (dizygotic) twin girl twins, Kian and Remee Hodgson. Described as a "one in a million" occurrence, experts explain that the girls inherited different genetic characteristics from their mixed race parents. One is fair- haired and light skinned, while the other has dark hair, eyes and skin.

12. 4.  "Half Identical" Twins: Polar Body Twins There are two types of twins, right? Dizygotic (fraternal) twins result when two eggs are fertilized. Monozygotic (identical) twins come from a single fertilized egg that splits. But what if the egg splits and then each half meets a sperm? That's the proposed theory for polar body or "half-identical" twins, twins who are very much alike but aren't a 100% DNA match. Although it seems to be a reasonable theory, there is no definitive test to confirm polar body twinning
5.  Boy/Girl Identical (Monozygotic) Twins
Identical (monozygotic) twins are always same gender because they form from a single zygote that contains either male (XY) or female (XX) sex chromosome. However, there have been a few reported cases of a genetic mutation in male twins where one twin loses an Y chromosome and develops as a female. The female twin would be afflicted with Turner's Syndrome, characterized by short stature and lack of ovarian development.
6.  Mirror Image Twins
Mirror image twins are monozygotic, twins that form from a single fertilized egg. When the split occurs late - more than a week after conception - the twins can develop reverse asymmetric features. This term is not really a type of twin, just an way to describe their physical features. For example, they may be right- and left-handed, have birthmarks on opposite sides of their body, or have hair whorls that swirl in opposite directions. In theory, if the twins faced each other, they would appear to be exact reflections of each other. About 25% of identical twins are mirror image twins.

13. 7.  Parasitic Twins A type of conjoined twins that develops asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin. A variation of parasitic twinning is fetus in fetus, where an abnormally formed mass of cells grows inside the body of its monozygotic twin. It survives during pregnancy, and even occasionally after birth, by tapping directly into the blood supply of the host twin.
8. Twins of Different Races Heteropaternal superfecundation can explain cases of fraternal (dizygotic) twins with differing racial characteristics.