Polar Body Twinning

Newborn twins
Polar Body Twinning. Stephen Swain Photography / Getty Images

When discussing types of twins, two types are widely recognized. Identical (or monozygotic) twins form from a single fertilized egg that splits; fraternal (or dizygotic) twins form from two distinct sperm/egg combinations. But polar body twinning is a term used to describe a theorized third type of twinning that occurs when an unfertilized ovum splits into unequal parts and then each part is subsequently fertilized by two separate sperm.

How Polar Body Twins Happen

The ova, the female sex cell, undergoes a process of division called meiosis to reproduce.  The process produces four granddaughter haploid cells, each with half a set of chromosomes. One of the four is the egg cell (oocyte) while the other three are called polar bodies. Usually, only the oocyte is available for fertilization, while the polar bodies fade away. But following the theory of polar body twinning, both the oocyte and a polar body would somehow become fertilized and develop, resulting in twins. 

Are My Twins Polar Body Twins? 

Polar body twinning is a concept or a theory. There are no observed cases of polar body twins and no method to identify or confirm or polar body twinning. While it may be interesting to speculate that your twins are similar, but not exactly identical, there's no way to know for sure if they are polar body twins. Where a zygosity test can confirm whether twins are monozygotic or dizygotic, companies that perform zygosity testing to determine twin type do not offer a test for polar body twins.

Are Polar Body Twins Alike? 

Where identical twins originate from the same zygote and are very genetically similar, fraternal  twins have about half of their genes in common, the same as any siblings. So it follows that polar body twins would have the same set of genes from their mother, but more variation in the genes from their father. In theory, polar body twins share about 75% of their genetic markers, less than identical twins but more than fraternal twins. Perhaps they look very similar, but not exactly alike. 

Polar body twinning is sometimes described as half identical twinning. It is distinguished from semi-identical twinning, where two sperm fertilize a single egg because the egg splits before fertilization. Although a case of semi-identical twinning has been confirmed, polar body twinning remains hypothetical.

View Article Sources
  • Australian Twin Registry. "What are The Types of Twinning?" Accessed February 12, 2016. http://www.twins.org.au/twins-and-twin-families/twin-resources/faq#twinning
  • Moskwinski, R., Ed. Twins to Quints: The Complete Manual for Parents of Multiple Birth Children. Harpeth House Publishing: 2002. Print.
  • Proactive Genetics. "What are Polar Body Twins?"Accessed February 12, 2016. http://www.proactivegenetics.com/faq-twins-genetics.html#56.
  • Wells, D. and Hillier, S.G. "Polar bodies: their biological mystery and clinical meaning." Molecular Human Reproduction. Volume 17, Issue 5, May 2011.