According to new research, the chemotherapy drug etoposide might be dangerous for ovaries developing in female fetuses. The findings, published in the journal BioMedCentral Cancer, changed the previously held view that most cancer drugs, when administered to pregnant women, are safe for their fetuses in the second and third trimesters.

The study results were concluded from mice experiments, but the researchers said the results are clear enough to extend the possible negative effects to human mothers and babies.

They found that the damage incurred by the fetal ovaries was dose-sensitive, and that the damage was worst when the ovaries hadn’t yet formed follicles. When the fetuses were given a dose of 50–200 ng ml−1 etoposide, researchers observed a “near complete absence of healthy follicles.” About 72 to 90 percent of healthy follicles disappeared if this dose was given, either from outright cell death or because affected germ cells went on to create unhealthy follicles.

When the ovaries of newly born female mice were treated with the same (or higher) doses of the chemo drug, there was significantly less damage because they were post-follicular.

Study author Norah Spears, PhD, said in statement that the results of this study are important not only for the specific findings about etoposide, but it raises questions about the potentially harmful effects of chemotherapy on fetuses that might not be known for significant amounts of time. The scope of many studies looking at safety in fetuses would have ended by the time the damage appears.

"This study suggests that chemotherapy treatment may have important longer term effects on the babies of women who undergo chemotherapy while pregnant which would only become apparent in adulthood. This is an issue that has not been explored until now. Studies looking at the effects of taking chemotherapy drugs during pregnancy have focused on the immediate effects, such as increased miscarriage rates or severe fetal abnormalities,” she said.

About one in 1,000 mothers could need chemotherapy treatment during pregnancy, meaning this new question about the continued safety of fetuses could have far-reaching effects.