Identical Fraternal Twins
January 19, 2012 by staff
Identical Fraternal Twins, Twins account for over 90 per cent of multiple births. Identical twins form when a single fertilised egg (ovum) splits in two. Fraternal twins develop from two eggs fertilised by two sperm, and are no more alike than individual brothers or sisters (siblings) born at different times.
Multiple births are more common than they were in the past, due to the advancing average age of mothers and the associated rise in assisted reproductive techniques, in particular the use of fertility drugs. Twins account for over 90 per cent of multiple births. There are two types of twins – identical (monozygotic) and fraternal (dizygotic).
To form identical twins, one fertilised egg (ovum) splits and develops two babies with exactly the same genetic information. This differs from fraternal twins, where two eggs (ova) are fertilised by two sperm and produce two genetically unique children, who are no more alike than individual siblings born at different times. 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 oestrogen than younger women, which means that their ovaries are stimulated to produce more than one egg at a time.
Number of previous pregnancies – the greater the number of pregnancies a woman has already had, the higher her odds of conceiving twins.
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.
Race – Black African women have the highest incidence of twins, while Asian women have the lowest.
Assisted reproductive techniques – many procedures rely on stimulating the ovaries with fertility drugs to produce eggs and, often, several eggs are released per ovulation.
Hormones secreted by the ovaries, and a small gland in the brain called the pituitary, control the menstrual cycle. The average cycle is around 28 days. After a menstrual period, rising levels of the hormone oestrogen help to thicken the lining of the womb (the endometrium) and release an egg from one of the ovaries (ovulation). If the egg is fertilised on its journey down the fallopian tube, it lodges in the thickened womb lining, starts dividing and evolves into an embryo.
Around one in three sets of twins is identical. This occurs because the fertilised egg divides in two while it is still a tiny collection of cells. The self-contained halves then develop into two babies, with exactly the same genetic information. Twins conceived from one egg and one sperm are called identical or ‘monozygotic’ (one-cell) twins. The biological mechanisms that prompt the single fertilised egg to split in two remain a mystery.
Approximately one quarter of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin.
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