Superfetation is an extremely rare phenomenon in which a woman becomes pregnant while already pregnant. This occurs when a second egg is fertilized by sperm and implants in the uterus while the woman is already pregnant with another fetus. The result is a pregnancy within a pregnancy, with two fetuses at different stages of development.

One remarkable example of superfetation occurred in 2009 when an Arkansas woman, Julia Grovenburg, became pregnant with a second baby while already pregnant with her first. This rare occurrence made headlines around the world and fascinated the medical community.

While superfetation is rare in humans, it is more common in certain animal species such as rabbits, hares, and kangaroos. This biological phenomenon has captured the interest of scientists and researchers seeking to understand the mechanisms that allow for such a unique occurrence.

References:

1. "Superfetation: A Rare and Controversial Phenomenon" - The Journal of Obstetrics and Gynaecology Research
2. "Rare Superfetation Pregnancy: A Case Report" - Obstetrics & Gynecology