Women with the BRCA1 gene mutation, which increases the risk of breast and ovarian cancer, may have fewer eggs in their ovaries and should not delay trying to get pregnant until their late 30s or 40s, a new study suggests.

Scottish and Australian researchers looked at both the BRCA1 and BRCA2 gene mutations, which are known to significantly increase a woman's risk of developing cancer of the breast, ovaries, fallopian tubes and peritoneum.

In recent years, these gene mutations have received much publicity due to celebrities such as Angelina Jolie revealing that they have undergone preventative procedures to reduce their risk of cancer. After discovering she had the BRCA1 gene mutation, Ms Jolie underwent a double mastectomy in 2013 and had her ovaries removed in 2015.

This latest study found that women with the BRCA1 mutation had lower concentrations of anti-Müllerian hormone (AMH), which is known to be a reliable marker of ovarian reserve, in other words, the ovaries' capacity to provide eggs that are capable of fertilisation.

Overall, AMH concentrations were around 25% lower in those with the BRCA1 mutation, although this effect was not seen in women with the BRCA2 mutation.

"This means that women in their mid-30s who carry the BRCA1 mutation have, on average, ovarian reserves similar to those of non-carriers who are two years older," explained Prof Kelly-Anne Phillips of the Peter MacCallum Cancer Centre in Melbourne.

The researchers emphasised that AMH is just one indicator of potential fertility and women with low AMH levels can go on to have children.

"The ability to conceive and carry a baby to full term is affected by many other factors as well, including egg quality and whether the fallopian tubes are unobstructed, neither of which are measured by AMH. Women with low AMH levels can sometimes still have a baby and, conversely, women with high AMH levels are sometimes unable to do so," they noted.

However, they said that the findings suggest that women who do have the BRCA1 gene mutation ‘should try to avoid delaying pregnancy until their late 30s or 40s when fertility is reduced anyway because of their age'.

"For women trying to conceive in their 20s, any difference in ovarian reserve between BRCA1 mutation carriers and non-carriers is unlikely to be of clinical significance," they added.

The study involved almost 700 women who were aged between 25 and 45. While none had cancer themselves, all had a family history of the disease. A total of 172 of the women were subsequently found to have the BRCA1 mutation, while 147 had the BRCA2 mutation.

Details of these findings are published in the journal, Human Reproduction.