Identical and non-identical twinning
Some introduction to the biology of twinning is important for the appreciation of multiples in school. There are two types of twins:
Non-identical, who stem from the fertilisation of two eggs and are no more alike genetically than ordinary brothers and sisters. The release of two or more eggs every month runs in families to some extent and also changes with age, as well as differing between ethnic groups. Such twins have separate placentas although sometimes these may be fused so closely they appear as one until there is detailed examination.
Identical, where the single fertilised egg splits sometime in the first 12 days after conception. If it splits in the first 3-4 days, the twins have separate placentas. In the most common situation, the split occurs 4-8 days after conception and the twins share the one placenta. A later split can lead to more complications and Siamese (conjoined) twins are those where the splitting process begins after 12 days.
(From D.A. Hay (1985) Essentials of Behaviour Genetics. Oxford:Blackwells, p218)
It is important to note that identical twins are only identical genetically and there are many physical and behavioural characteristics less determined by heredity where they may differ considerably and where non-identical twins may be very similar. Thus we shall use the more neutral terms DZ (for dizygotic or two-egg twins) instead of non-identical and MZ (for monozygotic or one-egg twins) instead of identical.
Higher multiples can be any combination of MZ and DZ. For example two eggs may be fertilised (making DZ twins) and then one developing embryo splits to form a set of MZ twins. It would be possible to have identical triplets where one early embryo splits and then one of these splits again.
We have such a family in Australia which interestingly resulted from fertility treatment. The doctors and initially the parents assumed the girls must be DZ, resulting from three eggs being fertilised. By the time they were three, the mother was pretty convinced they were MZ and blood tests confirmed this. There has been some discussion that the process of in vitro fertilisation may make splitting of the embryo and the formation of MZ twins more likely.