New Danish research has shown that women who take birth control pills have 29 to 52 percent lower ovary volume than women who are not on the Pill, with the biggest reductions in women aged 19 to 30.
The study, led by Kathrine Birch Petersen from the Copenhagen University Hospital in Denmark, also showed that the contraceptive pill might mask a woman's true fertility.
The research looked at 833 women between the ages of 19 and 46 who were attending the Fertility Assessment and Counselling Clinic in Copenhagen University Hospital between August 2011 and April 2014. It was triggered after the doctors realised many women on the Pill had "old looking" ovaries when it came to fertility.
"Young women typically have well-developed and well-functioning ovaries, while women approaching their menopause have smaller ovaries with reduced egg production," Birch Petersen told Science Nordic.
But this reduction in ovarian volume hasn't been shown to be dangerous, and Birch Petersen said that the ovaries appear to return to their normal size after the woman stops taking the pill.
The scientists also looked at levels of anti-Müllerian hormone (or AMH), a hormone that reflects the number of eggs a woman has, in the blood, as well as the number of early follicles in a woman's ovary (known as the antral follicle count or AFC amount) through ultrasound. Combined, these two measures are considered the best test for ovarian reserve and future reproductive lifespan - in other words, how long women can still have children for.
They found that women who were taking the contraceptive pill had 19% lower AMH and 16% lower AFC than in those who weren't taking the pill. Among the women who weren't taking the pill in this study, almost 30% had previously used it.
Even after statistically adjusting for age, body mass index, maternal age at menopause, maternal smoking during pregnancy and prematurity, AMH was still up to 30% lower and AFC up to 20% lower in Pill users than in non-Pill users.
The results were presented at the European Society of Human Reproduction and Embryology fertility conference (ESHRE conference) held in Munich at the start of July.
The research was part of a wider project to test how well doctors can measure future reproductive lifespan. It considered birth control pills and vaginal rings, but not contraceptive mini-pills or contraceptive implants.
"We expected to find an effect of the Pill," said Birch Petersen in a press release. "But during the project we were surprised at the quantified effect on ovarian reserve parameters as defined by anti-Müllerian hormone, antral follicle count and ovarian volume."
But don't get too alarmed - Birch Petersen explained that the main impact of the study is that doctors need to take into consideration whether a woman is on the Pill before trying to assess her future reproductive lifespan.
"It is not dangerous for women to take the pills, but if their fertility is being tested it is important to know that the pills can affect the results," Birch Petersen told Science Nordic. "The test results tend to indicate that the women is older and has fewer eggs than she may have in reality."
So it's not bad news for women who are on the Pill - in fact, recent Australian research has shown that in addition to preventing pregnancy and reducing the risk of ovarian cancer, the contraceptive pill can improve women's brain function.
And there's still a lot more we need to learn about how the contraceptive pill affects us.
"We do not believe that the Pill changes the ovaries in any permanent way," said Birch Petersen in a press release. "But we still need to know more about the recovery phase after women stop the Pill. The Pill is unlikely to change the basal ovarian biology of egg depletion - but it certainly changes the appearance of the ovaries and the secretion of AMH."