Marathon runner Jess Trengove is six months pregnant — but that has not stopped her from taking up a modified training regime with the goal of qualifying for the 2020 Tokyo Olympics.

Key points: Marathon runner Jess Trengove is due to give birth in November

Marathon runner Jess Trengove is due to give birth in November The dual Commonwealth Games bronze medallist is also training for the Tokyo Olympics

The dual Commonwealth Games bronze medallist is also training for the Tokyo Olympics She says she is working within her limits but admits qualification will be tough

Once her first baby is born in November, the South Australian athlete will have another six months to run a qualifying time for the games.

Trengove — who competed at the London Olympics in 2012 and in Rio in 2016 — is not deluding herself about the big challenge ahead, but has the drive and motivation to give it a go.

"It's a pretty tight window and certainly I'm well aware that it would be a huge feat to run a qualifying time," she said.

To make it to Tokyo, Trengove has to run a time under 2 hours, 28 minutes and 30 seconds, but will also have to overcome fierce competition.

Three Australian marathoners have already posted good times, including one who has run 2 hours, 24 minutes and 11 seconds.

"It's not just a matter of getting a good time, it's a matter of being in the top three out of the Australians as well," she said.

"It'll be a big challenge but I'm not thinking about it too much — I'm just going through each day trying to stay fit and healthy and once the baby is born, we'll look at the situation then.

"You just don't know how childbirth is going to be and you don't know how your baby will respond either."

Trengove's personal best is 2 hours, 25 minutes and 59 seconds.

The 32-year-old dual Commonwealth Games bronze medallist is used to pushing her body to the absolute limits — running 42 kilometres as fast as she can.

"I'll be able to build back into training and work from there. We don't have any real concrete targets at this point in time," she said.

"It's different in pregnancy in that any sign your body gives you, you really respect that and stop straight away."

'I'll just work within my limits'

Trengove is committing to an hour of cardio training each day and has two strengthening and conditioning sessions a week.

She is also continuing to go on walks.

"I was confident that I would have the drive and the motivation to train. It was more having the ability to read my body and know when to back off that's hard," she said.

"I'm so used to my coach setting me a session and doing all I can to complete that session."

Trengove finished 10th at the 2017 London Marathon. ( Supplied: Mark Kerrison/Alamy Live News )

But she admitted that training for a marathon was never easy, adding there were "always some little discomforts".

"You learn which ones you can push through but everything is very unknown in pregnancy," she said.

"I'm entering the third trimester now and anticipating there will be a lot of changes and certainly my tummy will grow a lot.

"I'll just work within my limits and try to stay healthy and set myself up for that next phase of giving birth."

Training would ease morning sickness, Trengove says

Trengove's last marathon was in October 2018.

It was after that race that she and her husband Dylan Stenson — who is also a runner — decided it was time to grow their family.

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"I just started to wind down and start recovering and set myself up to start a family," she said.

"I knew with the rigours of training, I would need to take it easy for a while. I found out I was pregnant in late February or early March and from that point, I backed off the running.

"I was probably doing a jog every second day when I could."

She said there was a time between 14 and 18 weeks when she suffered from pelvic pain and was unable to run.

"Then I started to introduce more cross and cardio training, which I'm familiar with when I've had injuries in the past," she said.

"To me, the elliptical [running machine] has been the most value to me because it replicates running — you don't have that pounding but your legs are moving in a similar way and I can still get my heart rate up."

Trengove has also been doing more work on a stationary bike and getting in the pool for a swim — something she intends to do more of in the third trimester.

"It was interesting in the first trimester, I'd wake up feeling a bit queasy, sort of that typical morning sickness, and your natural tendency would be to avoid exercise when you're feeling unwell," she said.

"But I actually found by getting out and going for a walk, getting fresh air and moving, it would ease the sickness.

"I didn't have too many days where I couldn't do any physical activity but sometimes if I'm on the elliptical and I might feel a bit crampy or uncomfortable, I'll just back off or stop that session for the day."

Trengove is getting her Adelaide home ready for her new arrival. ( ABC News: Simon Christie )

So far, though, she said that had not happened "too often".

"Through the second trimester, I didn't have the sickness so was able to be more active and comfortable within the limits of keeping my body temperature down," she said.

"There are some restrictions you have to keep in mind when pregnant and exercising so my intensity was a lot lower."

'Anything is possible', coach insists

Trengove's coach Adam Didyk said it was going to be a "big challenge" for Trengove to run a qualifying time after her pregnancy, but was still optimistic.

"Jess is a pretty tough cookie so if she can pull it together and her body cooperates, then anything is possible," he said.

"She's doing everything she can do at the moment."

Trengove with a young fan after winning bronze at the 2018 Commonwealth Games. ( Supplied: Jess Trengove )

Didyk said Trengove was training "within the realms of what she can do being pregnant — nothing too crazy".

"We're just giving her every opportunity to maintain a level of fitness to pick up once the baby is born," he said.

"Given that running is very much an impact sport, that's not completely possible with a body that's changing the way that a pregnant woman's does.