Multiple birth is where there is more than one fetus resulting from a single pregnancy. It is most applicable to placental species, with multiple births occurring in most kinds of mammals, with varying frequencies. Multiple pregnancy is the past tense. Twins refers to 2 offspring, and triplets refers to 3 offspring. In non-humans, the whole group may be referred to as a litter, and multiple births may be ore common than single births. Multiple births in humans are the exception, and can be exceptionally rare in the largest mammals.
Physiology
Each zygote (single fertilized egg) may produce a single embryo, or it may split into 2+ embryos, each carrying the same genetic material
Fetuses resulting from different zygotes are called fraternal, and share only 50% of their genetic material, as ordinary full siblings from separate births do
Fetuses resulting from the same zygote share 100% of their genetic material, and are hence called identical, and are always of the same sex (except in cases of sex reassignment, or much more rarely, Turner syndrome)
A multiple pregnancy may be the result of the fertilization of a single egg that then splits to create identical fetuses, or it may be the result of the fertilization of multiple eggs that create fraternal fetuses, or it may be a combination of these 2
Multiple birth siblings are:
Monozygotic, resulting from a single fertilized egg/zygote, splitting into 2 or more identical embryos (each carrying the same genes), which are thus the same gender
Polyzygotic (aka fraternal), result from multiple ova being ripened, and released in the same menstrual cycle by a woman's ovaries, which are then fertilized to grow into multiples no more genetically alike than ordinary siblings, sharing 50% of the genetic material
Dizygotic, is a combination of monozygotic and polyzygotic, where there is a multiple from 2 eggs, with one set of identical twins, and a 3rd non-identical sibling from a separate egg
Risks
Premature birth and low birth weight
Cerebral palsy
Incomplete separation
Mortality rate (stillbirth)
IVF
Mx
Selective reduction, which is the termination of 1+, but not all, of the fetuses in a multiple pregnancy
C section or vaginal delivery
Neonatal ICU
Epidemiology
Most common form, is twins
36% of twin births, and 77% of triplets+ resulted from contraception by assisted reproductive technology
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