Cesarean Sections are the most common surgery for women.
Hundreds are performed at hospitals every day, very often leading to the delivery of a perfectly healthy child.
The surgery has been fine-tuned to a fault.
"People have over time forgotten the magnitude of the risk," says Dr. Katherine Economy of Brigham and Women's Hospital.
In full-term babies that risk is breathing difficulty.
Doctors think there's something about the contractions of labor and going through the birth canal that allows extra fluid in the baby's lungs to be squeezed out.
"When they're born via C-section, they don't have that time, or that process and so many times, the lungs just aren't quite ready," notes. Dr. Economy.
Doctors have long assumed labor and delivery might be too rough for very tiny, pre-term babies having trouble growing in the uterus, but new research challenges that theory, comparing the health of pre-term babies born via the birth canal with those born via C-section.
"Babies being born via Cesarean delivery had a 30% higher risk of Respiratory Stress Syndrome," explains Janis Biermann of the March of Dimes.
C-sections are medically necessary in some cases, for breach babies and those whose heart rates plummet during labor, but maternal-fetal medicine specialists say the new research may re-shape the way doctors view tiny babies.
"We should be thinking about babies small for gestational as perhaps better able to tolerate labor than we originally thought," says Dr. Economy.
At least a third of all infants born in the U.S. are C-section babies.