Some deceases, including IP, are due to genes placed on the X chromosome and are therefore called ‘X linked deceases’.
Women have two X chromosomes whereas men only have one (they have a Y chromosome which makes them male) A woman with IP has one normal X chromosome whereas the other one bears the mutated gene.
When pregnant, she  transmits half her genes to the foetus. That’s why each time she is pregnant her chances of transmitting the X chromosome with the mutated gene are   50% whatever the sex of the foetus. Statistically half her daughters will inherit a normal X chromosome and will therefore be free from IP and the other half will inherit the X chromosome with the mutated gene and will suffer from IP like their mother.  Half her sons will inherit a normal X chromosome and therefore be normal and the other half will inherit the X chromosome with the mutated gene. Since a foetus needs at least one normal X chromosome to survive and boys have but one X chromosome, the foetus with the mutated gene will be miscarried or stillborn.