Q: What is the difference between identical or fraternal multiples?

A: 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.

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