The new baby should one day rule as monarch – whether it is a boy or a girl.
Thanks to the proposed shake-up of the rules of succession, its gender will not determine whether he or she wears the crown.
In October 2011, David Cameron announced that the 16 Commonwealth countries where the Queen is head of state had agreed to give female royals the same rights of succession as their brothers.
Under the ancient rules of male primogeniture, first born royal daughters in direct line to the throne were leapfrogged by their younger male siblings.
Moves towards constitutional change gathered pace in the wake of the duke and duchess’s wedding in April 2011 in anticipation they would produce offspring.
Mr Cameron had previously warned it would take time due to ongoing negotiations with the Commonwealth countries where the Queen is also head of state, but at the Commonwealth Heads of Government Meeting in October 2011 he confirmed the changes would go ahead.