If you have marfan syndrome, will they know it immediately after you are born?

Not if mild features: Marfan syndrome persons can have mild or moderate symptoms, so they can look pretty normal as babies. The physical features of taller height, long arms, long fingers, flat feet, high-arched palate, small lower jaw, chest deformities, abnormal back curves, etc..., become more obvious later as they grow up.

Likely Not: Diagnosing Marfan syndrome is based on a long list of specific criteria. Most infants will not exhibit the symptoms immediately after birth. So it is very unlikely to know immediately.