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Facts About Multiples

A Few Facts and Figures about Multiple Births

Types of Twins

Basically there are two different types of twins – monozygotic or identical (MZ) and dizygotic, fraternal and non-identical (DZ).

Monozygotic twins develop when a single egg is fertilised by a single sperm and at some stage in the first two weeks the developing embryo splits in two, with the result that two, genetically identical babies develop.

Dizygotic twins occur when two eggs are released at a single ovulation and are fertilised by two different sperm. These two fertilised eggs then implant independently in the uterus. Dizygotic twins share the same type of genetic relationships as non-twin siblings, hence the term fraternal.

MZ or DZ?

Male / female twin pairs, which make up approximately 1/3 of all twins births, are obviously dizygotic. Determining zygosity of same sex pairs can be more problematic. In the past, attending physicians have often diagnosed zygosity at the time of a twin birth based on the number of placentas found, the assumption made being that once placenta indicates that the pair is monozygotic, while two placentas are a sign that the pair must be dizygotic.

If the cleavage of the embryo that produces MZ twins occurs before about the fifth day of foetal life, there will be two chorions (outer membranes), two amnions (inner membranes) and two placentas (fig. A). If cleavage occurs between the fifth and tenth day (approx.) of foetal life, there will be one chorion, two amnions; and one placenta (fig. B). About 64% of MZ twins are of this type. If cleavage occurs between the tenth and the fourteenth day of development, the twins will have one chorion, one amnion, and one placenta (fig. D), but this happens in only around 4% of MZ twin births. If cleavage occurs after the fourteenth day, there is an increased risk that the twins will be conjoined or what is often called ‘Siamese’.

All DZ twins have two chorions (fig. A), as do some MZ twins. Pairs with separate chorions are called dichorionic, and in about 42% of twins of this type, the twins implant themselves so close together in the uterus that the placentas fuse, making it appear to the naked eye that there is only a single placenta (fig. B). Thus 42% of DZ twins would appear to have only one placenta at birth, while 19% of MZ twins will have two placentas.

The number of placentas at a twin birth is therefore almost useless as a guide to whether the twins are identical or fraternal.

Fig. A 

Separate placentas

2 chorions

2 amnions

Fig. B 

Fused placentas

2 chorions

2 amnions

Fig. C 

Single placenta

1 chorions

2 amnions

Fig. D 

Single placenta

1 chorions

1 amnions

The most objective way to assess zygosity is to use DNA fingerprinting, that is, testing the twins’ blood or another form of physical sample, such as cheek are usually reported as a likelihood ratio of the twins being MZ versus DZ. At the moment, DNA zygosity is specialised and expensive. There are a limited number of places in Australia which offer DNA testing as a service to twins. If you would like a contact for one of these companies, you can ask your local Multiple Birth Association or contact the Australian Twin Registry.