Monozygotic MZ twins, also called identical twins, occur when a single egg cell is fertilized by a single sperm cell. The resulting zygote splits into two very early in development, leading to the formation of two separate embryos. MZ twins occur in 3 to 4 per 1, births worldwide. Research suggests that most cases of MZ twinning are not caused by genetic factors.
However, a few families with a larger-than-expected number of MZ twins have been reported, which indicates that genetics may play a role. It is possible that genes involved in sticking cells together cell adhesion may contribute to MZ twinning, although this hypothesis has not been confirmed. Most of the time, the cause of MZ twinning is unknown. Dizygotic DZ twins, also called fraternal twins, occur when two egg cells are each fertilized by a different sperm cell in the same menstrual cycle.
DZ twins are about twice as common as MZ twins, and they are much more likely to run in families. Compared with the general population, women with a mother or sister who have had DZ twins are about twice as likely to have DZ twins themselves. DZ twinning is thought to be a result of hyperovulation, which is the release of more than one egg in a single menstrual cycle.
To explain how DZ twinning can run in families, researchers have looked for genetic factors that increase the chance of hyperovulation. Please enable scripts and reload this page. Turn on more accessible mode. Turn off more accessible mode. Skip Ribbon Commands. Skip to main content. Turn off Animations. Turn on Animations. Our Sponsors Log in Register. Log in Register.
In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac , called the amnion, in addition to sharing a placenta.
The technical name for this is monoamniotic twins. For example, a twin who receives less blood from a shared placenta might weigh less at birth. Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins. Women who are pregnant with twins sharing a placenta need to be checked more often than women with twins with separate placentas. Frequent checks can pick up early on any potential complications.
Twins sharing an inner sac monoamniotic are also at a higher risk of complications during pregnancy because of the chance that their umbilical cords might tangle and cut off their blood supply. These twins are checked even more closely. Medical professionals often recommend that these twins are born earlier than other types of twins. Medical professionals use ultrasound to work out how many placentas twins have. The earlier the ultrasound, the more accurately it can say how many placentas there are.
It gets harder to work out later in pregnancy. 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. The biological mechanisms that prompt the single fertilised egg to split in two remain a mystery.
0コメント