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. 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.
These babies will be no more alike than siblings born at separate times. The babies can be either the same sex or different sexes, with the odds roughly equal for each.
It is proposed that the egg splits in two, and each half is then fertilised by a different sperm. This theory is an attempt to explain why some fraternal twins look identical. Gestation for twins The normal length of gestation for a single baby is around 40 weeks. However, gestation for twins, either identical or fraternal, is usually around 38 weeks.
Since twins are usually premature, they are more likely to have lower birth weights. Prematurity is associated with increased risk of a number of disorders, including jaundice. Giving birth to twins Childbirth can give rise to complications when just one baby is present, so twins present extra potential for difficulties. It is advised that women carrying twins give birth in hospital, rather than at home.
The babies can be delivered vaginally, but caesarean section delivery may be considered a better alternative in some circumstances. Zygosity testing It is difficult to tell if twins are identical or fraternal at birth. Twins assigned different sexes at birth are always dizygotic fraternal.
Dizygotic twins form from two separate eggs that are fertilized by two separate sperm. They may be different sexes or the same sex. Monozygotic , or identical, twins are always assigned the same sex at birth.
While fraternal twins share similarities with each other, those similarities are comparable to siblings from separate pregnancies. So although they may look very much alike as siblings often do , fraternal twins are not identical. It's true that multiples can share a special connection that goes beyond that of ordinary siblings. Sometimes they will say or do the same thing at the same time. Some twins relate incredible stories of coincidence, where they think the same thoughts or sense each other's feelings.
Older studies on twin relationships have indicated that twins may share a stronger bond than non-twin siblings. However, more recent research has found no difference between twin and non-twin sibling relationships in terms of how they behaved toward one another. While there may be anecdotal evidence for a strong bond between multiples, there is not sufficient research to conclude that twin telepathy exists.
In times past, twins were often a surprise to their parents. Instead of one baby, two arrived! But now that ultrasound allows a sneak peek into the womb, there are rarely instances of a hidden twin. Despite the clarity of modern technology, multiples still manage to pull a few surprises and remain hidden from view during a portion of pregnancy.
This is more likely to happen if your ultrasound was done before 20 weeks, you didn't have an ultrasound, or if you have not received prenatal care.
According to the American Speech-Hearing-Language Association, what appears to be a secret language between twins may actually be a delay in phonology rules used to put speech sounds together.
Some twins use a secret language with one another and communicate effectively with others, like parents, so it's not always an indicator of a problem. However, secret twin language may be a risk factor for speech and language delays. If you are unsure whether your twins' secret language could indicate a speech delay, contact a speech-language therapist for an evaluation. They will be able to help you determine if your children's speech patterns are developmentally appropriate.
Anyone who knows a set of identical twins knows that they are alike in many ways, but also very unique individuals. Why is that? While twins often look very similar and may even share some of the same tastes and interests, they're also very different.
Identical twins form from a single egg and sperm combination and share the same genes, but environmental factors can influence differences in in twins' characters. Twins, by definition, are two offspring born together, right? Yet, some twins blow out their candles on different days.
Often, twins are born minutes apart—sometimes that interval just happens to be as a day or year changes over. But, other times, the span between each twin's birth can be even greater. When a pregnancy has a complication, such as preterm birth, a practice called delayed interval delivery may be used to stop labor after the birth of the first baby, allowing the other time to develop more fully before they are born.
This is very rare, but there have been case reports of delayed interval delivery of a month or more between births. Monozygotic twins occur when a single egg is fertilized by a single sperm. The zygote then splits in early pregnancy, forming identical twins.
Monozygotic twinning is not believed to be related to genes, although there are some families with an unusual number of identical twins, so it is possible that genes may play a role. Dizygotic twins, on the other hand, form when two eggs are fertilized by two sperm. In order for this to occur, a person has to release more than one egg during a cycle. This is known as hyperovulation, and it may run in some families.
If your parent or sibling had dizygotic twins, you are twice as likely to have twins yourself. The term refers to siblings who are born within 12 months of one another.
This is a slang term that stereotypes Irish culture. So, even if you intend to use it affectionately, it's better to avoid it. By definition, twins have the same gestational parent.
But it is possible for some twins to have different non-gestational parents. The technical name for this is dichorionic. Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different — for example, they might have different hair or eye colour.
Occasionally they look quite similar. Very rarely, fraternal twins share a placenta. These types of twins are called chimaeric twins. Sometimes a fertilised egg splits within a few days of conception to produce genetically identical twins.
Because these twins come from one zygote, they are also known as monozygotic. Identical twins are the same sex. About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas. The other two-thirds split after they attach to the wall of the womb.
As a result, they share a placenta. The technical name for this is monochorionic. 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.
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