A chemotherapy drug given to pregnant cancer patients could reduce the fertility of unborn baby girls, a new study suggests.
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.
Little is known about the longer-term effects of the drug on the unborn baby, however.
It is during the second and third trimesters that female germ cells form structures called 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 Professor 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.
Professor Adam 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 carried out in mice might relate to humans.
"We must also weigh any benefits of cancer treatment against these potential risks. These decisions should be made jointly between a woman and her specialist healthcare team."
Jacque Gerrard, director for England at the Royal College of Midwives, said: "Midwives are caring for women every day who are affected by cancer during pregnancy, and it's important that care given is based on the evidence.
"We hope that this is just the start of the research. Clearly more is needed if we are to provide safe care for our women with cancer in pregnancy to help improve outcomes for both mother and baby.
"The RCM is keen to support the midwifery profession in terms of improving care for women with cancer during pregnancy and beyond."