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

BBC: Teenage pregnancy is 'contagious'

Planned changes to the law of succession that end the tradition of a boy taking precedence over an elder sister are already de facto in effect, the British Cabinet Office said.

CNN: STORY HIGHLIGHTS

Planned changes to the law of succession that end the tradition of a boy taking precedence over an elder sister are already de facto in effect, the British Cabinet Office has said.

CNN: Duchess's 'slip' prompts speculation royal baby is a girl

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