Twins occur when two babies develop from a single fertilized egg. There are two main types of twins – identical and fraternal. Identical twins come from a single fertilized egg that splits into two embryos. They share 100% of their DNA and are always the same sex. Fraternal twins develop from two separate eggs that are fertilized by two separate sperm. They share around 50% of their DNA on average and can be the same or different sexes.
How common are twins?
It is estimated that around 3 in 100 births worldwide are twins. The twin birth rate varies around the world from 4 to 5 per 1000 births in Asia and Africa to 9 to 16 per 1000 in the United States and Europe. The increased use of fertility treatments in developed countries has contributed to a rise in twin births in recent decades.
Fraternal twins are more common than identical twins. Around 2/3 of twins are fraternal while 1/3 are identical. Some factors that increase the chance of having fraternal twins include:
- Family history – having a family history of fraternal twins
- Ethnicity – African ancestry increases the chance of twins
- Age – twins are more common in older women, especially over 30
- Fertility treatments – treatments like IVF often use multiple embryos
Identical twins occur randomly and do not have known genetic or environmental causes. The splitting of the embryo is a random event during early development.
How rare are different twin types?
While all types of twins are relatively uncommon compared to single births, some twin types are rarer than others:
Boy/girl twins
The most common type of twins are fraternal boy/girl twins. About 1 in 3 sets of fraternal twins will be boy/girl. This type of twinning occurs when two eggs are fertilized by an X sperm (producing a girl) and a Y sperm (producing a boy).
Boy/boy, girl/girl twins
Around 2/3 of fraternal twins will be same sex twins with either two boys or two girls. Fraternal boy/boy and girl/girl twinning happens when two eggs are fertilized by either two X sperm (girl/girl) or two Y sperm (boy/boy).
Mirror image identical twins
About 25% of identical twins develop as mirror images of each other. This happens when the fertilized egg splits later than usual, around 9-12 days after conception. Mirror image twins have symmetrical features and may have mirrored asymmetric features like opposite hair whorls.
Conjoined twins
Conjoined twins are identical twins who fail to fully separate in early development and are born physically connected. This is very rare, occurring in only 1 in 100,000 to 1 in 200,000 births. Female conjoined twins are more common than males.
Half identical twins
These extremely rare twins occur when a single egg is fertilized by two sperm and then splits. Half identical twins share 75% of their DNA. Only a few dozen cases have been documented worldwide.
Superfetation twins
These twins occur when a second egg is fertilized and implanted in the womb while a woman is already pregnant. This causes twins with different gestational ages. There are only a handful of confirmed cases.
Semi-identical twins
These rare twins develop when one egg splits and then both halves are fertilized by different sperm. Only two cases have ever been identified, in the United States and Australia.
Odds of rare twin types
Here is an overview of the estimated odds of some of the rarest types of twins:
| Twin Type | Odds |
|---|---|
| Identical twins | 1 in 250 births |
| Mirror image identical twins | 1 in 8 identical twin births |
| Conjoined twins | 1 in 100,000 to 1 in 200,000 births |
| Half identical twins | Extremely rare, less than 100 documented cases |
| Superfetation twins | Extremely rare, less than 10 documented cases |
| Semi-identical twins | Extremely rare, 2 documented cases |
As the table shows, some of the rarest types of twins like superfetation, semi-identical and half identical have odds of well over 100,000 to 1 or are so rare that their exact odds are unknown.
Factors affecting twinning odds
A few key factors influence the odds of having twins:
Maternal age
Older women are more likely to release multiple eggs during ovulation, increasing their chance of conceiving fraternal twins. After 35, twin rates steadily rise, peaking between 40-45 years.
Family history
Having a family history of fraternal twins significantly increases your odds of twins, especially in the mother’s family. However, family history does not affect the chance of identical twins.
Fertility assistance
Technology like IVF often uses multiple embryos, greatly increasing the odds of multiples like twins or triplets. Fertility drugs that stimulate ovulation also raise the chance of releasing multiple eggs.
Race/ethnicity
Some racial groups like African Americans have higher twinning rates. The exact cause is unknown but may be related to diet, genetics or body composition.
Conclusion
While any twin birth is relatively uncommon, fraternal twins of either sex are the most common. Identical twinning occurs randomly at a set rate. The rarest twin types like half identical, semi-identical and superfetation arise from very unusual fertilization events and have odds of 100,000 to 1 or greater.
Maternal age, family history, fertility treatments and race can influence the odds of twins by increasing the release of multiple eggs. However, the splitting of a single egg to produce identical twins remains a random and unexplained occurrence.
In summary, the rarest types of twins in order from most to least common are:
- Boy/girl fraternal twins
- Boy/boy and girl/girl fraternal twins
- Mirror image identical twins
- Conjoined identical twins
- Half identical twins
- Superfetation twins
- Semi-identical twins
So while any twins are a special occurrence, the rarest twin types are fascinating exceptions that give insight into the mysteries of human development.