In 2007, the most recent year studied, researchers found 192 out of every 100,000 pregnant and postpartum women received a cancer diagnosis - up from 112 per 100,000 women in 1994.
Researchers couldn't determine what was behind that increase in risk, but said it could be due in part to the older average age of expectant moms combined with better cancer detection.
Another explanation could be "the increased interaction with health services during pregnancy," said Christine Roberts, an obstetrics researcher at the University of Sydney who worked on the study.
Roberts said some doctors in her department had seen a few cases of expectant moms with cancer and wanted to know whether this was indicative of any increase in risk.
To try to answer that question, her group collected information from three large databases on births, cancer cases and hospital admissions in New South Wales, Australia. That included data on roughly 780,000 women who gave birth more than 1.3 million times between 1994 and 2008.
During the same period, there were about 1,800 new cancers diagnosed in moms-to-be and those who'd given birth within the last year. As diagnoses became more common over the years, pregnant women also got older, on average, the researchers noted in the obstetrics and gynecology journal BJOG.