Good genetics - health, height, intelligence all stem from 'good' stock
Good parenting - a secure, nurturing parenting style can enhance nature's benefits and produce a secure, confident child
The 'right' ethnic background, with a socially acceptable name - my name, for example, is stereotypically Irish and I've encountered racism and some truly ignorant fuckers in my 50-odd years in England
Wealth - a solid financial backing means a girl can try different sports, get extra tuition, if not a private education, a well filled savings account before she's even an adult and a solid financial education. Should there be any health issues, parents are more able to support their child and manage her condition
Social connections - with a well informed, well educated, socially connected family, a girl can get a head start in life
All this means a girl can have options to education and careers that many of us wouldn't/couldn't/haven't have had
Someone like The Honourable Kirstie Mary Allsopp, eldest daughter of the 6th Baron Hindlip (who was high up in Christie's the international auction house), would never have been heard of if she came from a middle class or working class background