GWEN IFILL: Today, the Supreme Court heard arguments in the case of Peggy Young, a former UPS driver who says the company discriminated against her when she was pregnant. UPS placed Young on unpaid leave for several months because she was unable to perform her required duties, they said.

Young's lawyers say the company's actions violated the Pregnancy Discrimination Act. Women's rights groups and members of Congress rallied outside the Supreme Court this morning to support Young.

But there are at least two sides to the argument.

Joining us to describe what happened inside the court today, Marcia Coyle of "The National Law Journal," Emily Martin, vice president and general counsel for the National Women's Law Center, and Karen Harned, executive director of the National Federation of Independent Business' Small Business Legal Center.

Marcia, I want to start with you and with the law. Let's look at this 1979 law, '8 law. I'm always getting that wrong. If I put on my glasses, I can see it.

MARCIA COYLE, The National Law Journal: OK.

GWEN IFILL: It says, "Discrimination on the basis of pregnancy is illegal sex discrimination, and pregnant women shall be treated the same for all employment-related purposes as other persons not so affected, but similar in their ability or inability to work."

Sounds pretty straightforward and pretty simple.

MARCIA COYLE: Simple, until you get into the Supreme Court and start arguing what the language means.

Today, the arguments really focus primarily on that second clause, how to treat pregnant workers. UPS has argued and it argued today that it has basically a pregnancy-blind policy. It offers accommodations to workers whose injuries occur or conditions develop on the job, not off the job.

So it's not singling out pregnant workers. They are being treated like all of UPS' other workers who have injuries or conditions that develop off the job. And it looked at that second clause and said, that's not a freestanding, independent claim to bring — to charge discrimination against UPS.

It is tied to the basic prohibition against pregnancy discrimination. Well, Ms. Young's attorney says, OK, let's look at the language of that clause again. It says nothing about on-the-job, off-the-job distinctions. It also doesn't speak to the cause or the source of the limitation on the worker.

Instead it says, you're to compare the pregnant worker limitation with non-pregnant workers who have similar limitations on their ability or inability to do the job. And also he claims that UPS doesn't have a pregnancy-blind policy because it does offer accommodations to workers, for example, who lose their Department of Transportation certificate that allows them to drive. And also it accommodates workers with conditions that are recognized by the Americans with Disabilities Act.

GWEN IFILL: Now, I gather this was a pretty lively set of arguments today at the court.

MARCIA COYLE: Very, very lively.

Justice Scalia and some other justices looked at the way Mrs. Young's attorney reads that clause. Justice Scalia used the phrase, it's so broad the way you're reading it that you're seeking most-favored nations treatment for pregnant workers.

Well, he was sort of expressing the concern that businesses that are supporting UPS say here, that any time an employer gives an accommodation to a worker with a limitation, regardless of how it happened or its severity, a pregnant worker is going to seek that same accommodation.

But, on the other side, Justices Ginsburg and Justice Kagan, for example, said that UPS' reading is so narrow that it is giving least-favored nation treatment to pregnant workers. And also the reading, according to Justice Kagan, it really makes the second clause redundant here, meaningless, and Congress could not have intended to do that.

GWEN IFILL: The administration had argued against bringing this case and now is on the other side. Right?

MARCIA COYLE: Right.

GWEN IFILL: How did that flip happen?

MARCIA COYLE: Well, it didn't argue because it didn't think the case wasn't worthy, but the administration did have a change of policy. For many years, it read that section the same way that UPS reads it.

In 2014, this year, over the summer, the Equal Employment Opportunity Commission issued new guidance. And its guidance on this supports Mrs. Young's interpretation of the law. And since the Department of Justice does follow EEOC guidance, that is how the administration is pursuing these cases now.

GWEN IFILL: Karen Harned, I want to start with the — a little bit of what the debate is. Is this a debate between employees' rights and employers' rights?

KAREN HARNED, National Federation of Independent Business: Well, it's really a debate between — we would first say it's a debate on what the law actually says, and we would be in the category that we definitely believe that if Ms. Young were to prevail, it would create a super protected class for the pregnant worker.

Our concern really is that there need to be limits. And if Ms. Young were to prevail, there wouldn't be the limits that you even see with the Americans with Disabilities Act, where if an accommodation would be an undue hardship on a business, that would be considered and you would also look at whether or not the accommodation was reasonable.

These are really questions that are better answered in Congress and the state legislatures, not through the Supreme Court, we think, rewriting the law.

GWEN IFILL: Emily Martin, are we just arguing this debate in the wrong place?

EMILY MARTIN, National Women's Law Center: Well, I think this case is critically important because it's really about whether the Pregnancy Discrimination Act means what it says.

And it's of critical importance for women around the country. Unfortunately, Peggy Young's story is not unique. At the National Women's Law Center, we hear again and again from women who have lost their job, have lost their paycheck because their employer refuses to make a simple accommodation like letting the cashier sit at a stool during an eight-hour shift late in her pregnancy.

And, as a result, women are being forced to choose between their jobs and a healthy pregnancy. And that's not a choice anybody should have to make.

GWEN IFILL: What is the difference — I'm going to ask the two of you, what's the difference between omitting coverage for a protected class or actively discriminating? What is your sense about that?

KAREN HARNED: Well, I think — and UPS argued this today — there are good business reasons why a company is going to have different classes of benefits for different classes of employees, part-time vs. full-time, in this instance, those injured on the job vs. those not, which is very common.

You need to be able to have that flexibility. And then, also, for the small employer in particular, their ability to backfill and make up for work lost by an employee is much more difficult when you're looking at a work force of 15 or 16 than with one of 200 or more.

GWEN IFILL: But do the anti-pregnancy discrimination laws that exist rule out allowing that protected class for this particular sort — subset of employees?

EMILY MARTIN: So, the Pregnancy Discrimination Act was passed for a very specific reason, to repudiate a previous Supreme Court case from 1976 where the Supreme Court said it's not sex discrimination for an employer to have a temporary disability insurance policy that covers all accidents and injuries, excludes pregnancy.

And Congress said very clearly, no, you have to treat pregnancy the way you treat other disabilities and injuries that can have an effect on a person's ability to work. That was the precise purpose of the Pregnancy Discrimination Act, to keep pregnant workers from being treated like second-class citizens in the workplace.

GWEN IFILL: So what you're saying is that the argument that Karen Harned makes is a second-class-citizens argument?

EMILY MARTIN: I think that what UPS is arguing is that the fact that they have found a way to accommodate people with on-the-job injuries, people with disabilities under the ADA, people who have lost their commercial driver's license, that should mean that they can accommodate pregnant workers, too. That's exactly what the Pregnancy Discrimination Act was intended to create.

GWEN IFILL: And it should be said that both UPS has changed its policy and the EEOC has changed its guidelines since this case came.

KAREN HARNED: Correct.

But, again, I would put to, for all employers, this one-size-fits-all is not going to work. When you're talking about small work force, maybe a small restaurant with only three servers working over the weekend, you lose one of those servers or some of their abilities to work, that's much harder for a small business owner to address.

That's why you need the balance that you get in things like the Americans with Disabilities Act, where you're also looking at undue hardship, what is a reasonable accommodation for those business operations to continue. Those are debates that need to happen in Congress, not at the court.

EMILY MARTIN: But, you know, the Pregnancy Discrimination Act only requires equal treatment. So it just requires the employer to do for the pregnant worker what it's already doing for another worker who has a similar inability to work.

GWEN IFILL: Marcia, one of the things I find most interesting about this case is the odd bedfellows who agree on one side of this. You seldom see pro-life and pro-choice people arguing the same — making the same case.

MARCIA COYLE: Right. I can't remember when that has happened in the past.

You have civil liberties group, women's rights group. As you said, I think something like 23 pro-life organizations have all joined to support Ms. Young.

GWEN IFILL: What is the common thread?

MARCIA COYLE: The need to ensure that pregnant women who are in the work force do get equal treatment and don't — and aren't forced, as Ms. Young was, to go on unpaid leave, where she also least her health insurance.

On the other side of UPS', as you would expect, are businesses large and small, and also Eagle Forum, which is a rather conservative organization.

GWEN IFILL: Women's group, that's right.

MARCIA COYLE: Right.

GWEN IFILL: OK.

Marcia Coyle, "National Law Journal," Emily Martin of the National Women's Law Center, and Karen Harned of the National Federation of Independent Business, thank you all very much.

EMILY MARTIN: Thank you.

KAREN HARNED: Thank you.

MARCIA COYLE: My pleasure.