There are many PS cases that don't fit these "rules": girl babies and later children also get the condition. There are other general characteristics of PS that also have many exceptions: it affects more Caucasians, children with blood groups A and O, children of mothers who were stressed during the last months of pregnancy.
If your baby is not keeping food down, vomits increasingly and powerfully, is losing weight and/or condition, is not passing motions and urine, suspect PS, get your baby tested, don't be put off by an unsuspecting and ill-informed doctor. On the internet there are many stories of children being treated only just before it's too late.