What?causes ?the patent ductus arteriosus?

Causes the PDA: The PDA is a critical vessel that allows the blood to bypass the lungs and get to the placenta while babys are still inside, in mom's uterus. The lungs are full of amniotic fluid and don't do any gas exchange (o2 delivery and co2 release) eveyone is born with a pda, it persists... Open (patent) most commonly in premature babies. There is no absolute answer as to why it stays open in some people.

It's necessary : The PDA is a normal bypass that allows the babies heart output to go directly to the body without passing through the lungs( which do nothing before birth).This reduces the work of the heart & allows the baby to use its energy to grow rather than do extra work. After breathing starts the increase in body oxygen level begins natural closure in most.