Twin Statistics

Identical Twin Rates

3.5 per 1000 ( 1 in 285) (this figure seems to be universal and random)

Conjoined Twins

1 in 50,000

More Than One Set?

If you have had fraternal twins already, your chance of having another set quadruples. This is because most women who conceive fraternals (without fertility drugs) show a pattern of releasing more than one egg per cycle. Your chance of a second set numerically, is about 1 in 3000.

The odds of having one set of identicals and one set of fraternals is 1 in 10,000.

If a woman has one set of identicals, her chance of having another is 1 in 70,000 or less.

The odds of having 3 sets of twins, of any kind, is 1 in 500,000.

Hellin's Law

Hellin's Law states that before the advent of fertility methods, the natural occurrence of multiples would be as follows:

Twins 1 in 90 live births
Triplets 1 in 8,100 live births
Quadruplets 1 in 729,000 live births
Quintuplets 1 in 65,610,000 live births

Those numbers have lowered to 1 in 38 for twins with fertility methods, as well as lowering the numbers for all other multiples. It is estimated that 60% of triplets are from fertility treatments, 90% of quadruplets are from fertility treatments, and 99% of quintuplets are due to fertility treatments.

 

If You Are A Twin (Or Have Twins In Your Family)

If you are a fraternal twin, you have a 1 in 17 chance of having twins. If you are identical, it doesn't affect your chances, and they remain from 1 in anywhere from 38 - 90, depending on if fertility drugs are used or not. If your mother is a fraternal twin, you may have a higher than average chance of twins. If you father is a twin, it doesn't affect your chances, as fraternal twinning is apparently carried down the maternal line.

If you have a parent that is an identical twin, your chances of having twins yourself are the same as anyone else.

Overall Twin Rates

White Americans/White Europeans: 14.5 in 1000 (1 in 69)
Black Americans: 1 in 60 (but as low as 1 in 40)
Japanese: 1 in 150
Chinese: 1 in 250
Mixed Asian: 1 in 125
Nigerian: 1 in 22
Norway, Denmark, Netherlands: 1 in 49
England: 1 in 76

These statistics are not infallible. The use of fertility treatments will cause the number of twin births to become higher, so they could be as high as 1 in 38 with the use of fertility drugs, to about 1 in 68 without (for all types of twins).

1 in 50 people is a fraternal twin in the USA, and 1 in 150 is an identical twin. 1/3 of all twins born in the US are identical, but in Japan the figure is 2/3.

Position of Twins

Twins can appear as both head first (both vertex), both breech, one breech and one vertex, one vertex and one transverse (sideways) and one breech and one transverse. These are a few numbers on what is most common:

Both vertex: Less than 50%
One vertex, one breech: 40% (usually the second twin is breech)
Both breech: 8%
Other: 2%

Identical Twins

Identical twins can have their own, or share, both the outer sac (chorion) and the inner sac (amnion):

    Number of Chorions Number of Amnions Number of Placentas When Split Occurs Commonality
    2 chorions 2 amnions 2 separate placenta 1-4 days after conception 25-30% of identicals
    2 chorions 2 amnions 2 fused placentas 1-4 days after conception
    1 chorion 2 amnions 1 placenta 4-9 days 65-70%, most common
    1 chorion 1 amnion 1 placenta 9-12 days 1-5%, rare

The only sure way to know if your twins are identical or fraternal (barring them being of different sexes or having different blood types) is a DNA test.

Did you Know?

The average length of pregnancy is 39-40 weeks for a singleton; 36 weeks for twins;

33-34 weeks for triplets; 29-30 weeks for quads; 28-29 weeks for quints.

Over 50% of twins are born before 37 weeks.

The average birthweight for a singleton is 7lbs 7ozs, and for a twin is 5lbs 5ozs.

93% of triplets and higher will be less than 5lbs 5ozs, compared to 6% of singletons.

About 35% of twins are left-handed, double the rate of the general population.

1 in 4 IVF births results in twins, 1 in 22 IVF births results in triplets.

Children of identical twins are genetically half siblings.

If identical twins marry identical twins, their children will legally be cousins, but genetically will be full siblings.

Other Links

http://proactivegenetics.com/
 
http://mypage.direct.ca/c/csamson/multiples/twinbasics.html