Probabilities in a sub-population

Complete population
The joint probabilities pxy and the marginal probabilities px and py all describe proportions in the complete population of (xy) pairs.
Sub-population
In contrast, it is sometimes meaningful to restrict attention to a subset of the (xy) pairs. Probabilities that relate to a sub-population are called conditional probabilities.

You should be careful to distinguish between px | y and py | x.

The probability of being pregnant, given that a randomly selected person is female would be fairly small. The probability of being female, given that a person is pregnant is 1.0 !!

Support and grief state after neonatal death