Superfetation is a rare and fascinating phenomenon in which a woman becomes pregnant with a second fetus while already pregnant with another. This extraordinary journey of conceiving again while already expecting is a marvel of the human reproductive system.

One example of superfetation occurred in 2009 when a woman named Julia Grovenburg from Arkansas, USA, became pregnant with a second baby despite already being pregnant with her first child. This rare occurrence made headlines and fascinated medical professionals and the general public alike.

While superfetation is extremely rare in humans, it is more commonly observed in certain animal species such as rabbits, rodents, and horses. These examples provide insight into the biological mechanisms that allow for the occurrence of superfetation.

References:

• "Superfetation: A Rare Pregnancy Phenomenon" - Medical News Today
• "A case of heteropaternal superfetation: fathered by two different men" - Journal of Forensic and Legal Medicine
• "Superfetation in Humans: A Review" - The Obstetrician & Gynaecologist