Twin Types

There are two basic types of twins: monozygotic, commonly referred to as identical twins and dizygotic, referred to as fraternal twins

Fraternal twins

Fraternal twins usually occur when two  fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins.
Dizygotic twins, like any other siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different to each other. Dizygotic twins may be a different sex or the same sex.
Studies show that there is a genetic basis for fraternal twinning. However, it is only the female partner that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum.

Identical twins

Identical twins occur when a single eggs is fertilized to form one zygote (monozygotic) which then divides into two separate embryos  The two embryos develop into fetuses sharing the same womb. Monozygotic twins are genetically identical and they are the same gender. Fine physical details such as fingerprints will differ. As they mature, identical twins often become less alike because of lifestyle choices or external influences..
  Dichorionic and diamniotic -These types of identical twins develop 2 separate placentas and 2 separate amniotic sacs (similar to fraternal or non-identical twins). They generally occur if the fertilized egg splits before 3 days after conception.

 Monochorionic and diamniotic - These identical twins share 1 placenta (monochorionic) but develop 2 separate fluid sacs (diamniotic). This is the most common type, accounting for around 75% of identical twin pregnancies and happens if the split occurs between days 3 and 8 after conception.

Monochorionic and monoamniotic - These twins share 1 placenta (monochorionic) and 1 sac (monoamniotic). This is the rarest type of identical twins, occurring if the split does not occur until 8 to 13 days after conception
The later in pregnancy that twinning occurs, the more structures will be shared. Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins

The likelihood of a single fertilisation resulting in identical twins is not a hereditary trait.The exact cause for the splitting of a zygote or embryo is unknown.
Mirror Image Twins
About 25% of identical twins are mirror image twins. Their hair falls in opposite directions, they have mirror image fingerprints and if one is right handed, the other is left handed.

Rate of Twin Births
Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba . The widespread use of fertility drugs causing has caused a major increase in multiple births. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Thus, approximately 6% of children born in the US in 2001 were twins.
Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly.

Higher Order Multiple Births
Sometimes multiple births may involve more than two fetuses. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, eight, octuplets, and so on. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of.

Are You Likely to Have Twins?
Predisposing factors
The cause of Monozygotic/ Identical twinning is unknown.
Dizygotic/Fraternal twin pregnancies are slightly more likely when the following factors are present in the woman:

• She is of African descent (especially Yoruba)
• Between the age of 30 and 40 years
• Greater than average height and weight
• Several previous pregnancies.
• Fertility treatments

Famous Parents of Twins

Famous Twins

Fun Facts
A peasant named Kirilow was presented to the empress of Russia in 1853 for the following reason. He had been married twice. His first wife bore fifty seven children, including four sets of quadruplets, seven sets of triplets, and two sets of twins. His second wife gave birth to fifteen children-six sets of twins and one set of triplets. At the time of his presentation to the empress, all seventy two of Kirilow's children were still alive.

Lots of Twins (Over 4 sets)

• Mrs. Mary Jonas (d. 1899) had 15 sets of boy/girl twins.
  
• A woman from Italy had 11 sets of fraternal twins in 11 years the last being in 1947 - not all of them survived.
• Marcella Mills-Bigcrow of Pine Ridge, South Dakota had 8 sets of twins,
  along with 8 single births. All the babies survived, and were born between the 1940's and 1960's.
  
• The Tremblay's of Quebec, Canada had 6 sets of twins in 8 years. Raymonde (her twin died at birth),
  
• Ronald & Ronaldo, Jacques & Jacqueline, Jules & Julien, Francine & Francois, Christien & Christiane.
  
• Mrs. Barbara Zulu of South Africa had 6 sets of twins in 7 years (1967-73, 3 sets of girls, 3 mixed)
  
• The Lecus family had 6 sets of twins and 19 children in total: Edna and Edward on April 3 1886,
  
• Charles and Martha April 27 1887, Elizabeth and Irene July 15 1888, Louise and Esther on August 8 1892,
  
• Agnes and David on January 3 1899 (Agnes died at age 7), and Emma and Viola in 1902. All were born in the Illinois/Michigan/Wisconsin area. Another daughter, Evangeline, had quadruplets that died at birth.
  
• Six sets of twins were born to Mrs. Harry Fifield of Connecticut.
  
• The Bockus family of Montreal had five sets, the last being born April 4 1845, a boy/girl set.

More Interesting Twin Facts

The World's Oldest Triplets Who Ever Lived
The longest-lived triplets ever recorded were Faith, Hope, and Charity Cardwell (USA) who were born on May 18, 1899. Faith died first on October 2, 1994 age 95 years 137 days.

The World's Oldest  Living Triplets
Marjory Skeaping (neé Scott), Sheila Botterill (neé Scott), and David Scott (UK) have been verified as the world's oldest living triplets. They were born in Edinburgh, SCOTLAND on May 19, 1920. Curiously, their Uncle, Aunt, and their Mother's First Cousin also celebrate their birthdays on May 19th.

The World's Oldest Living Quadruplets
The oldest quadruplets were The Ottmans (GERMANY). Adolf, Ann-Marie, Emma, and Elisabeth were born on May 5, 1912. All four quads lived to the age of 79. In November 2000, there were 992 sets of quadruplets worldwide. Identical quads are quite rare - only about 20 sets worldwide.

Tallest Female Twins in The World**
Heather & Heidi Burge -  6' 4 and 3/4
They were born in  1971, Palos Verdes, Calif 

Tallest Male Twins in the World**
Michael & James Lanier
7' 4 born in Troy Michigan

Shortest Twins in the World**
John & Greg Rice
2'10  - reside in West Palm Beach, Florida

Longest Lived Conjoined Twins**
Chang & Eng Bunker from Siam (now Thailand). They were born 5/11/1811 and
they passed away within 3 hours of each other at the age of 63 on 1/7/1874

Ronnie & Donnie Gaylon are the oldest living male conjoined twins. They are
47 years old and live in Ohio. For 36 years they traveled in shows,
carnivals and the circus. They retired in 1991.

Heaviest Twins**
Billy and Benny Crary - Hendersonville, NC
They were normal in size until they were 6 years old. In 1978 Billy was 743
pounds and Benny was 723 pounds. Their waist measurement was 7 ft.