multiple birth

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multiple birth,

bringing forth of more than one offspring at birth. Although many smaller mammals bear several young at a time, multiple births are relatively uncommon in humans and other primates. Twinning, the process that leads to the production of more than one offspring, results in twins, and with decreasing frequency, triplets, quadruplets, quintuplets, sextuplets, septuplets, and octuplets.

In the one-egg, or identical, type of twinning, a single fertilized ovum divides to form two complete organisms. Such twins are always of the same sex, are usually extraordinarily similar in physical appearance, and have identical blood-group types. Twinning to form one-egg identical twins usually takes place early in pregnancy. If considerable development has taken place before the twinning occurs, there may be an incomplete separation of the two embryos resulting in conjoined twinsconjoined twins,
congenitally united organisms that are complete or nearly complete individuals, historically known as Siamese twins. They develop from a single fertilized ovum that has divided imperfectly; complete division would produce identical twins, having the same sex and
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. Despite their similarities, identical twins have epigenetic differences that influence how their common DNA is used and expressed.

Fraternal twins, which are more common than identical twins, are those that develop from two separate ova, each fertilized by a sperm. Fraternal twins may or may not be of the same sex and need not resemble each other more than do any other two offspring of the same parents. In the United States twins occur once in approximately 40 births. In rare cases, non-identical embryos can fuse in the womb to produce a condition called chimerism, in which some of the individual's cells come from one of the embryos and the rest of the cells come from the other, genetically distinct embryo.

The incidence of multiple-egg births is in part genetically determined, varying according to race and family tendencies; and it is also influenced by external factors, i.e., the incidence increases with increasing age of the mother and the number of children she has already borne. One-egg, or identical, twinning occurs with the same frequency in all women, regardless of race, age, or other factors. There is evidence from comparative biology that deleterious factors in the environment of the newly fertilized ovum, such as a reduction in oxygen, increase the likelihood of one-egg twinning. Fertility drugsfertility drug,
any of a variety of substances used to increase the possibility of conception and successful pregnancy. Different methods are used to correct or circumvent the many different functional disorders of both males and females that can interfere with conception and
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 such as clomiphene, which are used when the cause of infertility is lack of released ova, sometimes cause several ova to be released and fertilized simultaneously. The use of these drugs has led to a rise in the incidence of multiple births, including sextuplets, septuplets, and octuplets.

References in periodicals archive ?
9 had a multiple birth - a slight fall from 2015 when the rate was 16.
Khyber Pakhtunkhwa has seen a dramatic rise in the number of multiple births.
Overall, the multiple birth rate has increased since 1976, when there were 9.
House members are seeking the granting of mandatory government support for triplets, quadruplets and other higher order multiple births to assist parents who are faced with the problem of raising several children all at the same time yet are not economically prepared for such a situation.
A third of mums of twins and triplets aren't getting access to an obstetrician specially trained in caring for multiple pregnancies, according to a new survey by the Twins & Multiple Births Association.
The number of multiple births from assisted reproductive technology continues to decline, with no more than 1% of women undergoing the procedures delivering triplets or higher orders of multiples in 2011.
However, the mortality rate for multiple births was five times higher than for singletons, with 20.
There is extensive evidence that twins and higher-order multiple births worldwide are associated with a substantially-higher risk of maternal and perinatal mortality and morbidity compared to singletons (1-4).
The parents discussed how multiple birth parents have to adjust their ideas of how to function in their previous daily activities.
Melanie, 30, said: "Where we live, there is no help for multiple birth babies, so it's great to know that the RVI is there.

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