Twin conception and zygosity testing advice
What happens when twins are conceived? There are two types of twins, identical and non-identical, depending on whether the twins result from the fertilisation of two separate eggs in one cycle or from a single fertilised egg dividing.
Dizygotic (DZ) is the term used for non-identical twins, although you will also hear them referred to as dizygous, binovular or fraternal.
DZ twins occur when two separate eggs are fertilised by two different sperm and there are two separate zygotes (formed when one egg is fertilised by one sperm) from the very start of the pregnancy.
Consequently, DZ twins are genetically no more alike than any set of siblings. About a third of non-identical twins are both girls, a third both boys and a third a boy and a girl.
Monozygotic (MZ) twins are identical, and you will also hear the terms monozygous or uniovular. MZ twins arise when a single zygote splits in two at some stage during the first 14 days after fertilisation. The resulting twins have identical genetic make-up and are of the same sex.
Are my twins identical?
Finding out whether twins are monozygotic (identical) or dizygotic (non-identical) is known as zygosity determination.
Reasons why parents want to discover the zygosity of their babies include:
- Determining the risk of inheriting certain genetic diseases
- Understanding the significance of similar or dissimilar growth and development in their twins
- Assessing the risk of having twins again (there is an increased risk if you have DZ twins)
- Reinforcing their resolve to treat their children as separate individuals, particularly if they are MZ
- To be able to answer other people's most commonly asked question: "Are your twins identical?"
In a third of cases, it's confirmed at birth that twins are non-identical because they are of different sexes. Similarly, it can be clear from ultrasound examination if they share a placenta and are therefore identical.
However, if the babies are the same sex but each have their own placenta, the only way to determine zygosity is by comparing their genetic make-up.
This is an easy and painless procedure, comparing their DNA. The most normal way is for a swab to be taken from inside the cheek of each baby.
Zygosity determination is not done routinely at birth, but parents can pay to have the test done. Costs vary, as can the reliability of the test, so speak to your doctor for further advice.
Content comes from the One Born Every Minute's book, Expecting Twins? A Complete Guide to Pregnancy, Birth & Your Twins' First Year
Last updated: 11 months ago