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 ?
Overall, the number of triplet births appeared to have peaked in 1998, with the number of triplets and higher number multiple births up a total of 400 percent since 1980 (Martin et al.
ING Re's Rosebud program has a staff of perinatal registered nurses who work directly with their high-risk insureds, such as pregnant women expecting multiple births.
During this time, the prevalence of congenital abnormalities arising from multiple births increased from 5.
The policy has worked and the multiple birth rate is down to 18 per cent for IVF clinics nationally while live birth rates have steadily risen.
Until now just one in three mothers have had access to specific parent education on having a multiple birth, which is really worrying as research shows that lack of access to such classes increases mothers' chances of developing post-natal depression.
However, empirical data suggest that competition among IVF clinics may not have a significant effect on multiple birth rates.
THE birth of octuplets will astound people worldwide, but multiple births are not as unusual as it may first appear.
Birmingham Women's Hospital earned praise in a survey by Tamba, the Twins and Multiple Births Association, for offering mums clear cut treatment and advice before they gave birth.
In a bid to cut the number of twins born as a result of fertility treatment, the Human Fertilisation and Embryology Authority is considering sanctions for clinics that produce too many multiple births.
TORONTO -- VON Peel, in partnership with Multiple Births Canada, is offering a program called "Good Beginnings".
Ryan Ruiz suffered multiple birth defects, including a missing right arm below his elbow, brain injury and cognitive deficits.

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