Signs of a twin conception: (7) Acardiac twin

In this case a twin develops abnormally so that major organs are missing, including the heart.
This is assumed to be associated with "polar body" twinning, where it is supposed that a single egg is fertilised twice, including a sperm entering the Polar Body:

See SPL image here:

This is a monozygotic (identical) twin pregnancy. 


Polar Body Twins
Most sources emphasize that "polar body" twins are a hypothesis that has not been proven. In essence, polar body twins are "half identical"-- the result of one egg fertilized by two different sperm. The resulting twins share the same genes from the mother but different genes from the father.

In the theory of polar body twinning, a woman ovulates, and as her egg matures, a portion of the egg called the first polar body separates from the egg. Because this polar body contains the same genetic material as the egg itself, theoretically it could be fertilized by a sperm cell (usually, however, the polar body just disintegrates). If both the egg and the polar body are fertilized by separate sperm and two babies develop from this event, polar body twins are formed.


In any case, whatever the cause, an acardic twin develops alongside the normal co-twin, and there are no major organs or even a head.

In the Dream of the Womb what could there be?  Some small part of yourself that remains perpetually undeveloped, a sense of being abnormal, without the ability to think or feel?  We have a lot to learn about what it is like to be a womb twn survivor when one's womb twin never developed a brain or a heart.