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Congratulations to you, the proud new parents or expectant parents of multiple birth children! This page is to welcome you to the fascinating world of multiples and to inform you of the many services that the Manitoba Families of Multiples organization has to offer.
A twin birth occurs approximately once in 90 births (non-medically assisted conceptions), triplets occur once in 8,100 pregnancies and quadruplets once in 729,000. There are two types of multiples, identical (monozygotic) and fraternal (dizygotic). Monozygotic twins result from the union of one egg and one sperm. The cause of monozygotic twins remains a mystery -- something delays the implantation and the division process of the fertilized egg so that the chromosomes, which carry the genes in a fixed quantity (46), double in number and the egg splits into two equal halves. Each twin develops from an identical half of the original fertilized egg. Monozygotic twins are always the same sex and have identical features; hair, eye color, blood groups, body scent, dental impressions, hair swirl patterns and ear print. In utero they may have one or two placentas, amnion and chorion sacs depending on the point at which division took place. The occurrence of two placentas during ultrasound does not determine whether or not your twins are identical or fraternal.
About one third of all twins are monozygotic, with an equal distribution of male-male and female-female pairs.
The more common type of twinning is dizygotic (fraternal). Two different sperm fertilize two eggs, which may be from one or two ovaries. Two separate unions or zygotes are formed, hence the term dizygotic. Dizygotic twins may or may not look alike, as with any other brother or sister comparison.
Approximately half of dizygotic twins are same sex pairs and half are male-female. The percentage of females increases with the number of multiples. For example, females are 48.4 percent of singletons, 49.2 percent of twins and 50.0 percent of triplets.
Triplets, quadruplets and other multiple births may occur as a combination of monozygotic and dizygotic twinning or as just one of these types. The Dionne quintuplets were monozygotic, the result of one egg splitting into five divisions after fertilization. Monozygotic triplets occur, but less frequently than from three fertilized eggs or a combination of monozygotic twins and one singleton.
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