Mount Sinai Hospital is investigating the death of a 4-month-old baby in its neonatal intensive care unit (NICU) during the recent nurses’ strike, according to a report.

The hospital began to formally look into the tragic January death of the infant, who had a severe heart condition, last week, according to a document reviewed by Gothamist. All infant deaths in the NICU automatically trigger an investigation.

Staff did not notice the baby’s low blood count for hours before the Jan. 11 death, which happened on the third day of the strike, according to the outlet’s sources.

Around 7,000 nurses walked off the job to protest working conditions. Sources who spoke to the outlet said non-union nurses were brought in to replace the striking workers and they did not have as much experience as the regular staff. They also claimed hospital executives didn’t properly prepare for the strike, leaving doctors with too much on their plate.

“These babies can get very sick very quickly … That’s why subtle things you see are important to report, and highly trained NICU nurses do that,” a source who worked in the NICU throughout the strike said.

During the strike, some babies received medicine hours later than they needed it, according to a source. 

According to the report, doctors were also expected to carry out nurses’ duties, including taking vitals and administering medicine.

“Parents felt like they couldn’t leave their child’s bedside during the strike,” the source told Gothamist.

Mother Lora Ribas told CNN how she didn’t leave her son — who was born prematurely and was on a ventilator due to his lungs being underdeveloped — for four days when the strike occurred.

“It’s scary to think that I can’t even go to the bathroom without me being concerned,” she said, noting that while the non-union nurses had been trying to compensate, they didn’t “really know my son” and had to learn where various supplies were around the unit.

Executives had been warned of the possibility of striking in September and issued a notice of strike in December before the nurses walked off the job.

Just a day after the baby died, the strike ended, with the union claiming the workers at the four hospitals in Manhattan and the Bronx won a “historic victory.”

The hospital declined to comment to The Post on Tuesday, citing an ongoing investigation. The hospital previously claimed the NICU was “appropriately staffed” during the strike.

The New York State Department of Health “takes instances of potential medical misconduct seriously,” its spokesperson told Gothamist.