Types of Twins

Like most people, Jamie and I thought twinning was as simple as identical or fraternal. As you may have already guessed it is not that simple. I have summarizied most of what you see below from the Wikipedia Twin article.
The first twin distinction is their zygosity: the degree of identity in the genome. There are five common variations

• Fraternal (dizygotic) twins occur when two separate eggs are fertilized at around the same time:
  
  • male-female (most common)
  • female-female
  • male-male
• Identical (monozygotic) twins occur when a single egg is fertilized forming a single zygote which splits:
  
• female-female
• male-male (least common)

The second twin distinction is the degree of separation of the twins in utero. Twins can share more than just their mother's womb. Fraternal twins are always Dichorionic-Diamniotic (DiDi), but when identical twins separate affects their type of separation.

Type	Description	Day
Dichorionic-Diamniotic	Twins share only the womb	Days 1-4
Monochorionic-Diamniotic	Twins share the same placenta	Days 4-8
Monochorionic-Monoamniotic	Twins share the same amnion	Days 9-12
Conjoined twins	Twins are physically connected	Days 12 or >

Jamie is having DiDi twins. They share the same womb, but nothing else. The ultrasound picture below shows the separation between the twins.

Regardless of the type, twins are rare, estimated to be approximately 1.9% of the world's population, with identical twins making up only 0.2% of the total population-and 8% of all twins.