Polar Body Twins Are a Rare Type of Twinning

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

If you have twins, are a twin, or know twins, the overwhelming likelihood is that they belong to one of the two main types: Identical (monozygotic) twins, which form from a single fertilized egg that splits, or fraternal (dizygotic) twins, which form from two distinct sperm/egg combinations. There are other types of twins that occur very rarely.

Then there's a type of twinning, known as polar body twinning, which is only theoretical, with no confirmed cases. (It's also called half-identical twinning.) Polar body twins would occur if 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 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. 

How Alike Are Polar Body Twins?

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 percent of their genetic markers, less than identical twins but more than fraternal twins. They might look very similar, but not exactly alike. 

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.

Other Rare Twin Types

While there are no confirmed cases of polar body twins, there are other rare types of twins that have actually been born. These include:

  • Conjoined Twins: Conjoined twins (once called Siamese twins) are identical twins that do not fully separate from each other due to the incomplete division of the fertilized egg. The babies are born with certain parts of their bodies connected and may share tissue, organs or limbs.
  • Semi-Identical Twins: Semi-identical twins occur when one egg splits before fertilization and is fertilized by two separate sperm. Semi-identical twinning is distinguished from Polar Body twinning because the egg was fertilized by two sperm before splitting.
  • Mirror Image Twins: Around 25 percent of identical twins are mirror-image twins. One looks like the mirror image of the other, so one might be left-handed and the other right-handed, for example, or they may have identical birthmarks on opposite sides of their body. Mirror image twins occur when the fertilized egg doesn't split until more than a week after conception.
  • Twins With Different Fathers: Known as heteropaternal superfecundation, fraternal twins with different fathers can happen when a woman has sexual intercourse with two different men on different occasions, but both of which occurred while the woman was ovulating and released two eggs. This situation can also occur when fraternal twins result from fertility treatments. In 1995, there was a case of Dutch twins conceived via in vitro fertilization (IVF). There was a mix-up in the lab which resulted in another man's sperm the lab to be mixed in with the father's, so each of two eggs released by the mother was fertilized with a different man's sperm. It became obvious when the twins were born and one baby had white skin while the other one had very dark skin.
1 Source
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  1. Mcnamara HC, Kane SC, Craig JM, Short RV, Umstad MP. A review of the mechanisms and evidence for typical and atypical twinning. Am J Obstet Gynecol. 2016;214(2):172-191. doi:10.1016/j.ajog.2015.10.930

By Pamela Prindle Fierro
 Pamela Prindle Fierro is the author of several parenting books and the mother of twin girls.