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Resources Centre / Twins Tips Introduction / Twins Tips / Conceiving Twins & Zygosity (Identical or Fraternal)
Conceiving Twins & Zygosity (Identical or Fraternal)
Multiple pregnancies have been on the rise in recent years with increasing numbers of twins and other types of multiples being born.
Generally. the growing use of fertility drugs and increased maternal age are attributed as the main reasons for this rise.

However. they are not the only reason why twins are born. Below we briefly discuss how multiple births occur and the determination of zygosity - are your multiples identical or fraternal ?

How do Twins and Multiple Births Occur ?
There are a number of different reasons why a multiple birth may occur:
Multiple eggs are released or there is more than one ovulation during your menstrual cycle. Two (or more) eggs are fertilized resulting in fraternal twins (non-identical). This can occur naturally or through the use of fertility drugs.
In the case of identical twins. one egg is released and fertilized but divides in two. This can happen with or without the aid of fertility drugs.
In cases of IVF or other reproductive technologies. there are usually 2 or occasionally more fertilized eggs put back into the uterus. The rationale is that not all of these will implant. though sometimes they do.

What is Zygosity Determination ?
Zygosity determination means finding out whether twins. triplets or more are identical (monozygotic - arising from one egg and one sperm) or non-identical (dizygotic/fraternal - from two separate fertilised eggs). This process is known as zygosity determination. (Download the charts to see diagrams explaining zygosity and chorionicity of twins)

Why is Zygosity Determination Important ?
It is only natural for parents to want to learn all they can about their babies. and with twins this includes their zygosity. Later. the twins themselves are usually keen to know. The reasons given by parents for wanting to know the zygosity of their twins include: -
  • For their own interest
  • To avoid embarrassment when asked the most common question. 'Are they identical?'
  • To reinforce their resolve to treat them as individuals if they are identical
  • To assess the risks of having twins again (there is an increased risk for women who have non-identical twins).
  • Furthermore. if the twins wish to participate in any Twins Studies in the future. knowledge of their zygosity will be essential.
How can Zygosity be Determined ?
Physical features
In a third of cases. determination is straightforward. because the children are of different sexes. therefore must be non-identical. Amongst twins of the same sex. by the time the children are around two years old. their zygosity may be quite clear from their physical features. Colour of hair and eyes. shape of ears. teeth eruption and formation. shape of hands and feet. and the pattern of growth can all give a good indication as to whether or not the twins are identical.

Examining the placenta
The placenta provides the answer in two-thirds of monozygotic twins. If the babies have a single outer membrane. (the chorion) they must be monozygotic. But one third of identical twins. those whose egg has split early before the placenta started to form. have two chorions with either a fused placenta. where the two placentas have grown together. or two separate placentas. These placentae are indistinguishable from those of dizygotic twins.

DNA testing using cells from the inside of the cheek
The most accurate method of determining zygosity is by the DNA probe method. The first stage in the analysis is to collect tiny amounts of DNA from inside each twins mouth. using a swab. Recently introduced molecular techniques are used to 'amplify' small sections from a number (currently 12) of specially chosen sites along the DNA.

The laboratory examine specific markers (representing individual regions of genetic code) present in the DNA of each twins. The regions have been chosen because most individuals in the population carry slightly different versions of DNA code at these sites. If only one or two of these markers were compared by chance. some non-identical (dizygotic/fraternal) twins. would give the same findings as identical (monozygotic) twins. which is why twelve of these diagnostic targets are looked at. Although dizygotic twins may share five marker patterns by chance. monozygotic twins will have the same pattern for all twelve.

The laboratory can determine if twins are identical with a reliability of 99.99%; in practice actual probabilities can be calculated and provided for each individual twin pair.

King's College Hospital offers a service to setermine zygosity from DNA extracted from cheek cells within the mouth. This is a simple procedure and full instructions are included with the cell collection kit. The email address for information is and the price is now 199 for twins, 299 for triplets and 399 for quadruplets. You can read about the test on the MBF website:

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