Multiple births are more common than they were in the past, due to the advancing average age of mothers and the associated rise in assisted reproductive techniques, in particular the use of fertility drugs. Twins account for over 90 per cent of multiple births. There are two types of twins – identical (monozygotic) and fraternal (dizygotic).
To form identical twins, one fertilised egg (ovum) splits and develops two babies with exactly the same genetic information. This differs from fraternal twins, where two eggs (ova) are fertilised by two sperm and produce two genetically unique children, who are no more alike than individual siblings born at different times. Twins are more or less equally likely to be female or male. Contrary to popular belief, the incidence of twins doesn’t skip generations.
Fraternal twins
Fraternal twins (commonly known as “non-identical twins”) usually occur when two fertilised eggs are implanted in the uterine wall at the same time that is when the mother releases two eggs and both become fertilized by two different sperms. The two eggs form two zygotes. and these twins are therefore also known as dizygotic.
Dizygotic twins. like any siblings. have a very small chance of having the exact same chromosone profile. but most likely have a number of different chromosones that distinguish them. Dizygotic twins may be a different sex or the same sex. just as with any other siblings. Like singleton siblings. they share 50% of their DNA.
Identical, or monozygotic, twins develop from a single egg/sperm combination that splits a few days after conception. Their DNA originates from a single source, thus their genetic makeup is the same and the characteristics that are determined by genetics will be similar. Monozygotic twins are always of the same gender, except in extremely rare cases of chromosomal defect.
Two-thirds of all twin births result in same sex fraternal twins and one-third are different sex fraternal twins. About two-thirds of all twin births are fraternal.
Identical twins
Identical twins occur when a single egg is fertilized by a single sperm to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The biological mechanisms that prompt the single fertilized egg to split in two remain a mystery. The two embryos develop into foetuses sharing the same womb. Depending on the stage at which the zygote divides. identical twins may share the same amnion (in which case they are known asmonoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). All monoamniotic twins are monochorionic.
On the other hand, fraternal, or dizygotic, multiples form when two separate eggs are fertilized by separate sperm in a single ovulation cycle. They are no more alike than any sibling set, sharing about 50% of their genetic markers in a unique combination of genes from both parents.
Normally, except the first two kinds, others are known to be quite rare to occur. All these twins, apart from similar faces have some varied characteristics.
Mirror image twins
These twins will often have “mirror images” of their features, such as hair whorls that run clockwise in one and counter clockwise in the other, a birthmark on the right shoulder of one and the left shoulder of the other, etc. There is no specific test for determining if twins are mirror-image. The determination is made by observation only.
Half-identical twins
These are the type of twins who share half their genes in common (from the mother) and the other half different (from the two sperms). They share some features of identical twins and some features of fraternal twins and thus are called half-identical twins.
Mixed chromosome twins
This occurs if two separate sperms fertilize two separate eggs which then fuse, producing individuals with different sets of chromosomes. Some have been identified to have more than one distinct red blood cell type and individuals who are both XX and XY (the sex chromosomes – XX being female and XY being male.) It is very rare, and less than 25 cases have been identified as yet.
Superfetation
Superfetation is the occurrence of another pregnancy during an ongoing pregnancy. It happens when there is fertilization of an ovule by a sperm cell while a conceptus is already present in the womb.
Superfecundation
Twins can have different fathers. One well known case was described in 1810 in the United States. A woman had both a white and a black lover, and she became pregnant and gave birth to twins, one white and the other black. Each twin had a different father. This is called superfecundation.
