Researchers found that a drug called etoposide can damage the development of ovary tissue in mice.
Etoposide is used to treat several types of cancer and is considered safe for use in the second and third trimester of pregnancy because it has a low risk of miscarriage and birth defects.
However, little is known about the longer-term effects of the drug on the unborn baby.
It is during the second and third trimesters that female germ cells form follicles that determine how many eggs she will be able to release in her lifetime.
Scientists at Edinburgh University found that treatment before the follicles had developed wiped out up to 90% of the germ cells, even at low doses.
Treatment after the follicles were developed had no significant adverse effects.
The team studied the effects of etoposide treatment on the development of mouse ovary tissue grown in the lab and said further research is needed to assess whether the drug has similar effects on human tissue.
About one in 1,000 pregnant women are diagnosed with cancer, leaving doctors and patients with difficult decisions to make to try and save the lives of both mother and baby.
Lead researcher Prof Norah Spears, from the university’s centre for integrative physiology, said: “If the results we have seen in these mouse studies are replicated in human tissue, it could mean that girls born to mums who are taking etoposide during pregnancy have a reduced fertility window.”
The study is published in the journal BMC Cancer and was funded by the Medical Research Council and the Biotechnology and Biological Sciences Research Council.
ProfAdam Balen, chairman of the British Fertility Society (BFS) and spokesman for the Royal College of Obstetricians and Gynaecologists (RCOG), said: “This is the first study to examine the long-term effects of chemotherapy during pregnancy and the results suggests that etoposide, which has been proven safe during the second and third trimesters of pregnancy, may have an impact on the finite store of eggs being created within their daughter’s ovaries whilst still in the womb.
“While this is important information for pregnant women who may need chemotherapy, it is too early to say the degree to which this study might relate to humans.”