For the Olympian Kara Goucher, the most difficult part of motherhood wasn’t resuming training just a week after childbirth in 2010 . It wasn’t even when her doctor told her she must choose: run 120 miles each week or breast-feed her son. Her body couldn’t do both.

The toughest moment was when Ms. Goucher learned that Nike would stop paying her until she started racing again. But she was already pregnant. So, she scheduled a half-marathon three months after she had her son, Colt. Then her son got dangerously ill. Ms. Goucher had to choose again: be with her son or prepare for the race that she hoped would restart her pay.

She kept training. “I felt like I had to leave him in the hospital, just to get out there and run, instead of being with him like a normal mom would,” Ms. Goucher said, crying at the memory. “I’ll never forgive myself for that.”

Nike acknowledged in a statement that some of its sponsored athletes have had their sponsorship payments reduced because of pregnancies. But the company says it changed its approach in 2018 so that athletes are no longer penalized. Nike declined to say if it wrote those changes into its contracts.

According to a 2019 Nike track and field contract shared with The Times, Nike can still reduce an athlete’s pay “for any reason” if the athlete doesn’t meet a specific performance threshold, for example a top five world ranking. There are no exceptions for childbirth, pregnancy or maternity.

Most people who spoke to The Times requested anonymity because they feared retribution, or had signed nondisclosure agreements, which may help explain why these arrangements have persisted.

Many American laws protect the rights of pregnant employees — they can’t be fired, for instance. But, since professional athletes are more like independent contractors, those protections don’t apply.