Flextime, childcare options and paid maternity leave are the keys issues for working mothers in the United States and what set companies apart as the best places to work, employment experts said on Tuesday. 

Top corporations are already providing those benefits and more to some of the 24 million mothers with children under the age of 18 in the U.S. workforce, but many other firms still do not have policies in place.

"Sadly, paid leave and flexibility is still innovative for a lot of employers in this country," said Jennifer Owens, the editorial director of "Working Mother' magazine, which released its list of the 100 best companies for working mothers on Tuesday.

"The smart companies are thinking about how they are serving their workforce and realize that their greatest resources are the people who work for them," she added in an interview.

When the annual list began 30 years ago, only five of the 30 best companies offered fully paid maternity leave and none had leave for new fathers or adoptive parents. Only seven had flextime.

More On This...

Today the 100 best companies offer an average of eight weeks of leave, as well as flextime, telecommuting and other benefits.

But across the country only 5 percent of companies have fully paid maternity leave, and 58 percent have some paid leave, according to a 2014 study by the Families and Work Institute.

Many companies also do not provide flextime and childcare assistance.

"Broadly in the United States we have come a long way in our working environment, but we still have a way to go," said Wanda Hope, chief diversity officer at health products and pharmaceutical company Johnson & Johnson.

"It will be wonderful when all companies support women in a way that allows them to really succeed and thrive at work."

FAMILY FRIENDLY CULTURE, MILK DELIVERY SERVICE

Johnson & Johnson and technology giant IBM Corp are the only two companies that have made the list for all 30 years.

Lindsay-Rae McIntyre, human resources vice president for IBM and a mother of three young children, attributes the company's long-standing on the list to its family friendly culture.

"Corporations that are the best for working mothers have to have a family-friendly culture at the core of who they are," she explained.

In addition to expanding its paid leave for new mothers and fathers, this month IBM is launching a concierge-type milk delivery service for nursing mothers who are away on business.

Owens said the most advanced companies are providing benefits that were not even considered 30 years ago, such as subsidized childcare at company headquarters, elder care at a center or at home and in-vitro fertilization and adoption assistance.

With 70 percent of mothers with children working, Owens believes companies cannot afford to ignore the needs of their employees, both men and women.

"What these companies are doing to support working mothers, typically helps everyone," she added. "To retain the best employees they need to be thinking about the big questions, surveying them, finding out what the challenges are."

The full list of the 100 best companies can be found here.