(CNN) — Four female pilots are filing discrimination charges against Frontier Airlines, alleging the company's policies on pregnancy and nursing fail to accommodate breastfeeding requirements. The Denver-based pilots, who are all employed at Frontier, claim that upon returning from maternity leave, the airline did not provide proper accommodations to pump breast milk. In their charge, Shannon Kiedrowski, Brandy Beck, Erin Zielinski and Randi Freyer say their employer did not give them a designated location to breastfeed at the airport, nor on the aircraft. The group also says Frontier did not offer temporary work reassignments in the final stages of their pregnancies, when they were unable to fly. Instead, they were forced to take eight to 10 weeks of unpaid leave, they claim.
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"There's a very clear law that requires employers to provide breaks and non-bathroom locations for employees to express breast milk," said Galen Sherwin, senior staff attorney with the ACLU Women's Rights Project.
Frontier Airlines challenged the claims.
"Our policies and practices comply with all federal and state laws as well as with the relevant provisions of the collective bargaining agreement between Frontier and its pilots group," the airline said. "While there are many workplaces that might allow for nursing mothers to express breast milk during a break from work activities, the duties of a commercial airline pilot present unique circumstances. We have made good-faith efforts to identify and provide rooms and other secure locations for use by breastfeeding pilots during their duty travel."
According to the ACLU, the pilots have faced a number of setbacks as a result of the company's policies. Coupled with the demand of flight schedules, the pilots were often made to delay pumping, resulting in pain and discomfort. Three of the women say they developed breast tissue infections, because the airline's policy did not allow them to pump on a regular schedule. Kiedrowski says she was disciplined when a co-pilot complained that she pumped on the plane.
First Officer Erin Zielinski, who had planned to breastfeed her child for one year, said she quit nursing early when her milk supply dried up.
In 2014, while flying a regular Denver to Los Angeles route, Zielinski said she had 15 minutes to pump in the plane lavatory, between flight preparation, flying the aircraft and readying the plane for a quick turnaround back to Denver.
"I love my job as a pilot so much, except for this issue," Zielinski said. "We don't want future moms to have to go through this. We want a better policy for everyone going forward. There are more and more female pilots being hired, including at Frontier. We don't want anyone to have to choose between flying and breastfeeding."
Some of the women also claim they suffered financial stress from unpaid leave before giving birth.
In a contract negotiated by Frontier Airline Pilots Association in 2007, pilots can work through the 32nd week of pregnancy, provided they have medical clearance. The pilot will then request maternity leave. Once the pilot has given birth, maternity leave is granted for up to 120 days -- and must be taken in concurrence with the federal Family and Medical Leave Act -- paid from sick and/or vacation time.
The group "sought information, support, and accommodations from Frontier, but were met with indifference or outright hostility," according to the ACLU.