The Efforts

 “Twingenuity [is] the inventive skill of combining the efforts of young twins in pursuit of a common goal (i.e. room destruction, sleep evasion, identity confusion, etc.)” (Anonymous)

In a society where there are assisted reproductive techniques and increased use of fertility drugs, multiple births are on the rise. Statistics reveal that twins account for over 90% of multiple births. Twins are more or less equally likely to be female or male. Contrary to popular belief, the incidence of twins doesn’t skip generations. 

Some women are more likely than others to give birth to twins. The factors that increase the odds include:

Advancing age of the mother – Women in their 30s and 40s have higher levels of the sex hormone estrogen than younger women, which means that their ovaries are stimulated to produce more than one egg at a time.

Assisted reproductive techniques – Many procedures rely on stimulating the ovaries with fertility drugs to produce eggs and, often several eggs are released per ovulation.

Heredity – A woman is more likely to conceive fraternal twins if she is a fraternal twin, has already had fraternal twins, or has siblings who are fraternal twins.

Number of previous pregnancies – The greater the number of pregnancies a woman has already had the higher her odds of conceiving twins.

Race – Black African women have the highest incidence of twins while Asian women have the lowest.

There are further intriguing classifications of twins apart from the common fraternal and identical twins. These ten different types of twins include:

Conjoined twins: This refers to monozygotic twins whose body parts are joined together at their birth. It may be the tissues, limbs, or body organs. These types of twins occur when single zygote of multiples or identical twins fails to fully separate from each other as a result of an incomplete or delayed division of the fertilized ovum (which should essentially occur within the first eight days since the inception).

Fraternal twins: Also referred to as non-identical twins, fraternal twins are also quite common. These different types of twins usually occur when two fertilized eggs are lodged onto the uterine wall at the same time.

These independent eggs are formed as a result of fertilization of two ova by two different sperms and results in two zygotes, which gives the phrase “dizygotic twins.” Such twins have different genetic makeup. They share approximately 50% of their DNA, and like normal siblings; they do not necessarily have to be the same sex.

Heteropaternal Superfecundation: Remember the dizygotic twins we mentioned earlier where two different sperms fertilize two different eggs? This is also the case for this particular type of twins. The difference is, the fertilization of the eggs by sperms happens in separate incidences of sexual intercourse.

This explains how twins sometimes end up having different fathers since if a woman has sex with different partners, the eggs will be fertilized by different sperms and at different intervals. For example, the widely debated instance in 1810 in the United States where a woman with both a white and a black lover became pregnant and gave birth to twins, one white and the other black and each with a different father.

Identical twins: Identical twins are one of the most common types of twins. They are genetically identical. They occur when a single egg is equally fertilized by a single sperm to form one zygote, which later splits into two separate embryos. The splitting is expected to happen between 4 to 8 days after fertilization, which results in monochorionic-diamniotic twins.

Mirror-image twins: As the name suggests, these are identical twins that are literal reflections of each other. These twins may share the same DNA or even possess almost matching fingerprints. Apart from these similarities, they may also have different features especially if the split occurred late. For example, if one is left-handed, the other may be right-handed. Their organs may be on opposite sides of the body. Statistics also suggest that mirror-image twins constitute 25% of all identical twins.

Monozygotic twins: These types of twins are always of the same gender since they form a single zygote containing either XX female or XY male sex chromosomes. In cases where a genital mutation has occurred, the male may lose the Y chromosome and develop into a female, who may suffer from Turner’s syndrome as they grow up, which manifests through short stature and absence of ovarian development.

Parasitic twins: While conjoined twins tend to share some organs, parasitic twins are dependent on each other, with the smaller and less developed one depending on the larger and more developed twin.

There are some variations within this category; namely, chimera where one of the twins borrows some body parts from the other, and fetus in fetu where an abnormal mass of cells develop undetected within the body of the monozygotic twin. It feeds on the blood supply of the host twin and may even be discovered in adulthood.

Polar-body twins: Imagine what would happen if a single egg split into two before fertilization and each half was later fertilized by separate sperm. You would end up with half-identical twins otherwise known as polar-body twins, who are 75% identical.

Semi-identical twins: These rare types of twins were identified back in 2007 in a pair of 3-year-old twins who were both identical, but with only half of their paternal genes. They were developed when two sperms fertilized a single egg. One of the twins is a hermaphrodite with both male and female reproductive organs while the other has only male organs.

Superfetation: This is a case where twins are conceived at different times. Normally, a woman’s ovulation ceases once an egg is fertilized. However, an egg may be released and fertilized when the woman is already pregnant, which is the difference in the time the twins are born.

“Not even identical twins can have the exact same experiences, and their brains are not wired the same way.” (John Medina) [i]

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[i] Adapted from:

·       “10 Different Types of Twins and Their Characteristics “by incubar

·       “5 Easy Ways to Tell Your Newborn Twin Babies Apart” by Sara Novak

·       “DNA” by Wikipedia

·       “Twins - identical and fraternal” by BetterHealth Channel

The acronym DNA stands for “Deoxyribonucleic acid.” 

British twins (left to right),  Lucy and Maria Alymer