thirdborn

1. not comparableBorn as the third child to parent or family.

   Mary, the thirdborn Bennett child in the novel Pride and Prejudice, receives the least attention.

2. not comparableThird of a particular type to be born to a parent or family.

   Prince Arthur was Queen Victoria's thirdborn son and her seventh child overall.

plural thirdborns

1. The third child born to a parent or family.

   After correcting for factors that may affect scores, including parents’ education level, maternal age at birth and family size, the researchers found that eldest children scored an average of 103.2, about 3 percent higher than second children (100.3) and 4 percent higher than thirdborns (99.0).

   2007 June 22, Benedict Carey, “Research Finds Firstborns Gain the Higher I.Q.”, in New York Times