Final answer:
A partial molar pregnancy usually arises from polyspermy, which is when multiple sperm fertilize one egg, resulting in a genetically abnormal embryo with an incorrect number of chromosomes.
Step-by-step explanation:
A partial molar pregnancy is caused by an abnormality of the fertilized egg which can occur due to polyspermy, where multiple sperm fertilize a single egg. This irregularity leads to an incorrect number of chromosomes in the developing embryo. Unlike a tubal pregnancy, which is related to physical blockages in the fallopian tubes, a molar pregnancy involves a genetic error during the fertilization process. This condition is characterized by the development of abnormal tissue within the uterus, which can resemble a mass of cysts (a mole).
Properly regulated fertilization ensures that only one sperm enters the egg to prevent polyspermy. However, when this mechanism fails, the resulting conceptus has too many chromosomes and is genetically nonviable, potentially leading to a molar pregnancy. This differs from situations where fertilization does not occur at all, such as when opting for in vitro fertilization or surgical interventions like ligation of the fallopian tubes. In those cases, pregnancy, and thus molar pregnancy, is intentionally prevented.