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, and early genetic mutations typically result in some small genetic differences between identical twins.

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.

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References in periodicals archive ?
But there was no additional widening after multiple pregnancies.
Accuracy of ultrasound estimation of fetal weight in multiple pregnancies. Ultrasound Obstet Gynecol 1995; 6:349-52.
[USA] July 26 ( ANI ): Multiple pregnancies might make women's cells age more quickly, a new study suggests.
On the other hand, the primary reason most often cited for preferring twins was "avoiding a new IVF attempt" (61.6%), followed by "I like the idea of having twins" (27.3%), "avoiding the waiting list" (5.8%), and "in my opinion, the rate and severity of complications in multiple pregnancies are low" (5.2%).
Multiple pregnancies involve substantial health risks for mothers and babies, as well as costs to the NHS.
UCD's Professor Michael Turner said the main reason for the spike is the increase in multiple pregnancies following Assisted Reproduction or IVF in Ireland.
Dr Styliani Andronikou, laboratory director and consultant embryologist at the Dubai Gynaecology and Fertility Centre, said: "The transfer of embryos is strictly as per the clinical history of the patient to reduce the occurrence of multiple pregnancies. We do not encourage multiple pregnancies and that is not always safe or healthy.
Their topics include physical principles and bioeffects of first trimester ultrasound, fetal biometry and pregnancy dating in the first trimester, multiple pregnancies in the first trimester, the fetal gastrointestinal system, and the placenta and umbilical cord.
"There is a risk of multiple pregnancies when more than one embryo is placed into the womb.
The test can detect multiple pregnancies, problems with the placenta or fetal heart, or whether the fetus is growing too slowly.

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