When placed in car seats, babies who are only a week or two premature can have breathing problems similar to those faced by infants born much earlier, a new study suggests.

The majority of preemies are born at 34 to 36 weeks’ gestation, when they’re considered late pre-term, and less at risk for respiratory problems that are common among earlier arrivals with less-developed lungs, researchers note in Pediatrics. The American Academy of Pediatrics recommends screening all preemies to ensure they can sit in the semi-reclining position in car seats without breathing problems, but the study team notes that this test is often overlooked for late pre-term babies.

For the study, researchers examined car seat screening results for 918 late pre-term babies. Overall, 4.6% of these babies failed the test, meaning they could risk potentially fatal breathing difficulties by riding in a car seat.

“Infants who spent time in both the neonatal intensive care unit (NICU) and the newborn nursery had the highest risk of failing, which is important for doctors to know when preparing these babies for discharge home,” said senior study author Dr. Natalie Davis of the University of Maryland School of Medicine in Baltimore.