It's not known why this is but it appears that body composition can influence the chance of having twins. Several studies have shown that for tall women cm and over , the relative chance of having twins is between 1.
Mothers of twins also have a higher body mass index BMI compared to mothers of single children - a BMI of less than 20 is associated with a lower probability of twinning while a BMI of 30 or more is associated with a higher likelihood.
There is also evidence that the chance of having twins increases the later a mother gives birth. Experts think this happens because of a change in hormonal signals sent between pituitary gland and the ovary.
In the UK, the odds of having any kind of twins at any age is one in 63 but these odds reduce to one in 46 once a woman is older than 35, and one in 18 if a woman conceives after the age of IE 11 is not supported. For an optimal experience visit our site on another browser. Politics Covid U. News World Opinion Business. 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. Factors that increase the odds of having twins 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. Fertilisation 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. 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. So genetic recombination helps prevent siblings from looking the same, but that's not its only function.
The process is important for the survival of a species. By reassembling or recombining genes, it ensures that harmful genes are not automatically passed down to future generations. So Tom and Jen's, and Tim and Jan's children wouldn't look the same, but their situation would still be a little strange: even though Tom and Jen's children would be cousins to Tim and Jan's children, all the children would be as closely related — or have the same genetic relationship — as brothers and sisters.
What's more, some of the children might also be identical twins: this is because identical twin pairs like Tom and Jen, and Tim and Jan carry genes that mean they have a higher likelihood than non-twin couples of having children that are identical twins. Tags: fertility-and-infertility , science-and-technology , biology , genetics , dna. Email ABC Science. By clicking 'Send to a friend' you agree ABC Online is not responsible for the content contained in your email message.
Skip to navigation Skip to content. This site is being redeveloped.
0コメント