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.

• 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

• Premature birth and low birth weight
• Cerebral palsy
• Incomplete separation
• Mortality rate (stillbirth)
• IVF

• 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

• Most common form, is twins
• 36% of twin births, and 77% of triplets+ resulted from contraception by assisted reproductive technology