Rebecca and Isabelle Sutherland have known each other since they were children.

The couple, now in their late 20s, married and experienced the wonder of being new parents to their four-month-old son, Bailey.

But before they grew into a family of three, the mothers had a conversation that became a pivotal moment in their lives.

Isabelle had been increasingly troubled by information she was reluctant to share with her partner, causing Rebecca to fear the worst.

"I was worried because it was not long after we got engaged," Rebecca said.

She was convinced Isabelle was having second thoughts about their engagement. She guessed her fiance could be about to leave her for someone else.

"I just cornered her on the couch and was like, 'You're going to tell me whatever's going on'," Rebecca said.

Isabelle explained: "We just kind of broke down and I said, 'Look, I'm trans,' and Bec said, 'Oh, is that all?'"

Rebecca said her fiance's revelation prompted her to make her own. "Oh, I guess I have to tell her I'm bi now. I was keeping it a secret."

'We'll just give it a try'

The Sutherlands on their wedding day, before Isabelle's transition. ( Supplied: Rebecca Sutherland )

The pair said the experience strengthened their relationship. But it was not until an unexpected conception and miscarriage — Rebecca had been on contraception — that they discussed parenthood.

"We were never quite sure if we were going to try for a child before I started medically transitioning, or use in vitro fertilisation after the fact," Isabelle said.

Isabelle began freezing her sperm in case she went ahead with a gender transition.

"Ultimately, we decided, 'You know what, we'll just give it a try for a couple of months to see how it goes before I start hormones. Maybe something will happen, maybe it won't, we'll play it by ear'," Isabelle said.

Rebecca was pregnant two weeks later.

The Sutherlands said they were cautiously optimistic about how friends, family and the wider community would react to their news.

However, the support they received was overwhelming, from parents — who had been subtly pressuring for grandchildren — to conservative and religious friends.

Marriage vote a 'difficult time'

Rebecca had a difficult pregnancy with Bailey, tainted by pre-eclampsia and severe nausea. ( Supplied: Rebecca Sutherland )

Once the pregnancy reached the second trimester, Isabelle saw a doctor to begin a medical gender transition.

That meant both Rebecca and Isabelle's bodies and lives were changing drastically at the same time.

Rebecca had a painful pregnancy, tainted by pre-eclampsia and hyperemesis gravidarum, which caused severe nausea.

And Isabelle's decision to change gender was at times a difficult journey.

"I opted to go to a psychologist because I came out in the [same-sex marriage] plebiscite and there was a lot of hate in the media," Isabelle said.

"I work in the media and I saw it constantly. I was bombarded. I got quite depressed and realised I needed to see someone to deal with it."

The difficult pregnancy was followed by a complicated birth.

Rebecca and Isabelle with their newborn boy. ( Supplied: Rebecca Sutherland )

Bailey was born six weeks' premature and needed to stay in hospital in an incubator.

But while he was small for his age, he displayed no other problems.

The Sutherlands said they and their medical team were shocked by how soon he started to roll himself over.

"The doctor's note on his discharge was 'baby thinks he's full term, better send him home'," Isabelle said.

'Who's mum?'

The three are now thriving, but their modern family has also encountered a small number of hurdles.

The Sutherlands said there were times during the pregnancy, before they changed hospitals, when some medical staff seemed judgmental and "hung up" on Bailey having two biological mothers.

But the vast majority of encounters were positive, and usually involved people doting on their newborn.

Isabelle transitioned to being female while Rebecca was pregnant with their son. ( ABC News: David Sciasci )

"People come up to us all the time [saying] 'oh, isn't he adorable' and have a chat about babies, and have a chat in general, and the fact that we're two mums almost never factors into it."

They say the most common question they are asked is: "Who's mum?" They answer: both of them.

Rebecca, who is writing a memoir, hopes telling her family's story will help transgender parents be accepted and better understood, to the point where, one day, "no-one bats an eyelid".