Dana Vollmer, the Olympic swimmer racing while six months pregnant

Vollmer (centre, in green) was glad to have found a swimming costume that "holds everything in"

With a little more than 1,000 days to go until the 2020 Tokyo Olympics begin, preparation - even at this stage - is key.

Such is the case with American swimmer Dana Vollmer, who claimed her seventh Olympic medal in Rio last year.

But she has had to adapt her preparation for Tokyo to cater for, in her words, the "bowling ball" in her stomach.

In her first race since Rio on Thursday, Vollmer was six months pregnant.

The first obstacle was finding a swimming costume that worked for her - up from the size 26 she wore in Rio to a size 32 at Thursday's race in Mesa, Arizona.

"It kind of holds everything in," she said, in an interview on the Team USA website. "We had to go up a few sizes to hold the belly."

Vollmer's son Arlen checking in on his brother-to-be

Vollmer raced with her doctor's permission, and has amended her training programme while pregnant by doing more strength training.

In an interview before the race with ESPN, she said training was a welcome distraction.

"As hard as people think this is, the race is only 30 seconds long as opposed to the entire day I spend holding and chasing around a 35-pound two-year-old," she said. "This will feel like a break."

After winning gold in the 100m butterfly in the 2012 London Olympics, Vollmer took time off to have her first child, son Arlen, and returned in time to qualify for Rio.

But this time around, she has made the decision to continue training. Baby number two, a boy, is due in July.

"Putting the health of the baby first doesn't just mean sitting on the couch," the 29-year-old said.

Why it's OK to run when you are pregnant

Vollmer isn't the first woman to race while heavily pregnant - in June 2014, Alysia Montano competed in the 800m quarter finals of the US track and field championships while eight months pregnant.

Plenty of women have also taken part in the Olympics while pregnant, though none did in Rio, due to concerns over the effect of the Zika virus on unborn children.

The website of the US Swimming Masters, an organisation helping promote the sport, advises that women can carry on swimming even while heavily pregnant, but that each case is different.

Vollmer had teased her fans by saying her suit would indicate the sex of the baby - pink for a girl, green for a boy (she eventually swam in green)

Competing in the 50 metres freestyle only three months before giving birth did present one particular challenge for Vollmer.

"I don't think I've ever done a 50 where I took four or five breaths," she said. "A 50 felt long for the first time in my life."

In the end, Vollmer finished 55th in the preliminary round in Mesa, with a time of 27.59 seconds (last year, she swam the same race in 25 seconds).

Not that her time was a problem.

"Time didn't matter, place didn't matter," she said. "I've loved being here. I've loved seeing all my teammates, all the people from Rio. The race felt great."