Assume that the probability of either gender of a newborn is 50-50.
Then ...
The probability of the first child being a girl is 0.5 .
The probability of the second child being a girl is 0.5 .
The probability of the third child being a girl is 0.5 .
The probability of the fourth child being a girl is 0.5 .
The probability of all four being girls is (0.5) x (0.5) x (0.5) x (0.5) =
(0.5)⁴ = 0.0625 = 6.25% = 1/16 .