17. What is the difference between identical or fraternal multiples?

The zygosity of multiples may not be as simple as scientists once believed. While most multiples are defined as being either identical (mono-zygotic) or fraternal (di-zygotic) researchers and health care providers are still discovering and learning about an array of possible variations that include half-identical multiples, polar body multiples, and others. For the purposes of this question, we will focus on mono-zygotic and di-zygotic zygosities.

• Identical multiples (mono-zygotic) are the result of a single fertilized egg splitting into two, three, four, or more babies and will almost always result in the same-gender multiples who share nearly identical DNA. (On very rare occasions an embryo with three sex chromosomes, XXY, will split and may result in monozygotic twins of the opposite gender.) Fewer than 20% of most higher-order pregnancies involve identical multiples. The babies may appear in distinct sacs and with separate placentas, or they may share the same placenta (mono-chorionic: meaning one chorion or placenta). A small percentage of mono-chorionic multiples may also share the same amniotic sac (mono-amniotic). An even smaller percentage of mono-zygotic multiples are conjoined twins.

• Fraternal multiples (not identical) are the most prevalent zygosity in higher-order multiples and make-up around 80% of triplets or more. Di-zygotic (fraternal twins), tri-zygotic (fraternal triplets), tetra-zygotic (fraternal quadruplets), or penta-zygotic (fraternal quintuplets) multiples occur when two or more embryos implant from separate fertilized eggs and can result in the same or different genders. Though the placenta of fraternal multiples may appear to fuse as the pregnancy progresses due to proximity, fraternal multiples do not “share” a placenta or amniotic sac. The DNA of fraternal multiples could be as similar or different as any other genetic sibling.

In higher-order multiples, zygosity types can occur in a wide variety of combinations. For example, triplets can consist of two identical (mono-zygotic) boys and a fraternal (di-zygotic) girl, or quadruplets could consist of a set of identical triplet (mono-zygotic) girls with a fraternal (di-zygotic) sister.

See the MOST Statistics page for the odds of having identical triplets or more.

MOST also offers our book Expecting Multiples: A Comprehensive Guide covering a variety of topics related to pregnancy and delivery of higher-order multiples.

For more information on the percentage of identical and fraternal zygosity in higher-order multiples, visit the MOST Supertwins Statistics page.

MOST also has volunteers whose families include indentical triplets, identical quadruplets, and higher-order multiples with mixed zygosity (ex: quintuplets with a set of identical twins, quadruplets who are two sets of identical twins, etc.)

See National Geographic's In the Womb: Multiples video for more on zygosity.

Other interesting resources on zygosity and twinning:

• Zygosity Chart
• Identical versus Fraternal Quiz
• Unusual Types of Multiples
• The Alphabet Soup of Twin Biology
• FAQ on Zygosity

Families of Conjoined Twins - A PRIVATE YAHOO Group offering support and friendship group for families with or expecting conjoined twins and for families with children who were born conjoined.

Please Note:
MOST provides these FAQs for informational purposes and cautions visitors not to use the content below to make treatment decisions without personally consulting a qualified health care provider. Reuse of this content without proper citation is a violation of copyright. To obtain permission to use Supertwins 101 content contact MOST.

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Copyright MOST 2005      Updated 1/18/10