The diagnosis came after a routine blood test to check her glucose levels for gestational diabetes, and left Mrs Cannell stunned.

''It was disbelief because I was feeling fine, I had no symptoms,'' she said.

Doctors quickly reassured Mrs Cannell that her baby was unlikely to be affected by the blood cancer, partly because they did not share the same blood.

But her own situation was more perilous, with her cancer so aggressive that it could have killed her if she did not begin urgent treatment.

Mrs Cannell was referred to the Royal Melbourne Hospital, which works closely with the Royal Women's Hospital to manage such rare cases.

While delaying her own treatment or giving birth prematurely would once have been her only options, a growing body of evidence is showing that chemotherapy after the first trimester does not present a risk to the unborn child. In Mrs Cannell's case, she had a modified combination of drugs and close monitoring before giving birth to baby Jack, now a healthy six-week-old, at 34 weeks.

Royal Melbourne Hospital haematologist Kylie Mason said the hospital only treated a couple of pregnant women with blood cancers each year - but doctors were becoming increasingly confident about how to treat them without harming unborn children.

An international study of 70 children exposed to chemotherapy in the womb, published in The Lancet, found their development, IQ, and general health were the same as in the general population at a median age of two.

Those who were born early had lower IQs, which was thought to be connected to their prematurity rather than the chemotherapy.

Dr Mason said treating pregnant women for cancer was complicated by the stage of their pregnancy and how urgently they needed treatment. In Mrs Cannell's case, she said the worst outcome would have been losing the baby. ''It's a really difficult situation because you've got the welfare of the unborn child as well as the mother, and a lot of the chemotherapy drugs we use routinely are toxic to the foetus.

''We're worried about severe growth retardation or chemotherapy affecting the baby's blood counts, so we have to modify the [drugs] very carefully so they do not harm the baby.

''Every time we approach a patient with an aggressive malignancy it's an individual situation … with Julia we were very pleased with the outcome.''

Mrs Cannell's acute lymphoblastic leukaemia is in remission, and she is having more intensive chemotherapy treatment that will continue for the rest of this year.

Daughters Hannah, 4, and Lucy, 3, adore their baby brother Jack, and know that their mother has a blood disease and needs lots of treatment and will take a while to get better.

Mrs Cannell said her cancer had a ''40 to 60 per cent survival rate, something like that - whatever the percentage is, I'm planning to be in it''.

With the support of husband Scott, she is caring for her three children and grateful that she was never faced with the prospect of terminating an early-stage pregnancy to save her own life. ''I'm so glad I never had to make the decision of whether to have him or not.''