What are twins?

The number of twins, or multiples, being born is actually on the rise due to the increase in use of fertility treatments such as IVF as people wait longer to have children. The number of twins surviving early births is also increasing due to improved medical knowledge.

However, twins are still a relatively rare occurrence making up only around two per cent of the living world’s population. Within this, monozygotic twins (from one ovum) make up around eight per cent with dizygotic (from two ovum) seen to be far more common.

While there is no known reason for the occurrence of the split of the ovum that causes monozygotic twins, the chances of having twins is thought to be affected by several different factors. It is believed twins ‘run in the family’, often seeming to skip generations, while the age, weight, height, race and even diet of the mother are thought to potentially impact the chances of conceiving dizygotic twins. Also, if the mother is going through fertility treatments, she is much more likely to become pregnant with multiples.

It will become apparent quite early on that a mother is carrying twins as this is often picked up during early ultrasound scans. There can be other indications such as increased weight gain or extreme fatigue. Although twins are often born entirely healthy and go on to develop without problems later in life due to medical advances, twins can be premature and smaller than single births due to space constrictions within the womb during development.

Strange but true

There are many stories of identical twins being separated at birth and then growing up to lead very similar lives. One example described in the 1980 January edition of Reader’s Digest tells of two twins separated at birth, both named James, who both pursued law- enforcement training and had a talent for carpentry. One named his son James Alan, and the other named his James Allan and both named their dogs Toy. There were also the Mowforth twins, two identical brothers who lived 80 miles apart in the UK, dying of exactly the same symptoms on the same night within hours of each other.


Twins are a relatively rare occurrence, making up only around two per cent of the living world’s population.

Multiple pregnancies, multiple problems

There are many difficulties with twin pregnancies – mainly due to the limited size of the mother’s womb. Multiple pregnancies rarely reach full term due to these limits, twins averaging at around 37 weeks. Also, because of the lack of space and eggs splitting in the womb, further complications such as conjoined twins can occur. Conjoined twins can be a problem dependant on where they’re joined. If it is by a vital organ or bone structure, one or both may die following birth as they grow – or during an operation to separate them.

It is also suspected that as many as one in eight pregnancies may have started out as a potential multiple birth, but one or more of the foetuses does not develop through to full term.

Twins inside the womb

Twins inside the womb

From studying identical, monozygotic twins, we can attempt to decipher the level of impact environment has on an individual and the influence genes have. As the genetics of the individuals would be identical, we can say that differences displayed between two MZ twins are likely to be down to environmental influences.

Some of the most interesting studies look at twins that have been separated at birth, often when individuals have been adopted by different parents. Often we see a similar IQ and personality displayed, whether or not they grow up together, but even these and other lifestyle choices can vary dependant on environment.

Ultimately, it is hard to draw firm conclusions from twin studies as they will be an unrepresentatively small sample within a much larger population and we often find that both environment and genetics interact to influence an individual’s development.

Formation of identical and fraternal twins

Monozygotic (MZ), or identical, twins are formed by the egg splitting soon after fertilisation, and from those identical split groups of cells, two separate foetuses will start to grow. Monozygotic twins are therefore genetically identical and will be the same sex, except when mutations or very rare syndromes occur during gestation. No reason is known for the occurrence of the split of the ovum, and the father has no influence over whether identical twins are produced.

Dizygotic (DZ) twins, however, are produced when the female’s ovaries release two ovum and both are fertilised and implanted in the womb wall. They can be known as fraternal twins as genetically they are likely to only be as similar as siblings. They will also have separate placentas, where MZ twins will share one, as they are entirely separate to each other – they are just sharing the womb during gestation. This kind of twin is far more common.

Formation of identical and fraternal twins

Formation of identical and fraternal twins

Top 5 facts about multiple births

1. 12 foetus world record

Although sadly this pregnancy did not reach full term and no babies survived, a woman in Argentina was observed to have 12 foetuses inside her womb from natural conception.

2. Nonuplets exist

A Malaysian mother gave birth to five boys and four girls in March 1999 – none survived over six hours. Another set of nonuplets suffered a similar fate in Sydney in 1971.

3. Race affects conception

With regard to DZ twins, there is seen to be a higher prevalence of these twins in black Africans, and a lower incidence than to be expected in Oriental mothers.

4. Most children

The highest recorded number of children had by one woman is the first wife of Feodor Vassilyev, who lived in the 1700s. She gave birth to an incredible 69 children.

5. Quad gestation

With regard to quadruplets, the average gestation period is just 32 weeks – that’s eight weeks shorter than usual, primarily due to the size restriction of the womb.

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