Professor Propper said an elder sister with a baby could also influence the decision to keep a baby.

BBC: Teenage pregnancy is 'contagious'

Her elder sister Ejegayehu was an Olympic silver medalist in 2004, while younger sister Genzebe was the 2009 world junior cross country champion and a younger brother Dejene is a promising 800m runner.

CNN: Born to run: Ethiopia's golden girl Dibaba

Serena's elder sister Venus is also paired with an American.

CNN: Tennis legends tip Murray for grand slam glory at U.S. Open