Taking birth control pills may make women's eggs "look old" in a sense, at least based on two tests of fertility, a new study has found.
In younger women taking the pill, hormone levels associated with their ability to make mature, healthy eggs, are more akin to those of older women, according to the study. Women on the pill also have fewer structures in their ovaries that can mature into viable eggs.
However, the new results don't imply that the birth control pill prematurely ages women's eggs, the researchers said.
Instead, the findings suggest the pill obscures a woman's underlying reproductive status, so tests that are typically done to assess women's fertility shouldn't be done on women taking the pill, said Dr. Lubna Pal, director of the menopause and polycystic ovarian syndrome programs at Yale University in Connecticut.
"Women should not be freaking out that they are losing their eggs" if they're taking birth control, said Pal, who was not involved in the study. "These [tests] are yardsticks that should be applied only in the context of fertility assessments." [7 Surprising Facts About the Pill]
A woman's "ovarian reserve" is a measure that predicts how well her ovaries produce mature oocytes, or eggs, that can be fertilized. As women age, their ovarian reserve diminishes, leading to fewer eggs, and fewer that reach a mature stage.
Most doctors assess ovarian reserve by measuring the levels of anti-Mullerian hormone (AMH) in the blood, as well as by conducting a vaginal ultrasound to count the number of early-stage structures called follicles in the ovary. Together, these two markers are strongly correlated with how a woman's ovaries are aging. (Women who are close to menopause tend to have lower values on these tests.)
In the new study, researchers looked at both markers of ovarian reserve in 833 Danish women between ages 18 and 46, including some who used oral contraceptives.
Those women who were taking the pill had 19 percent lower levels of AMH, and 16 percent fewer early-stage follicles. Their ovaries were also much smaller than those of women who were not taking the pill.
The effect held even when researchers adjusted for other factors known to affect those markers, such as women's body mass index (BMI), and whether they smoked.
The findings make sense from a biological perspective, Pal said.
The hormone levels the researchers measured generally increase as eggs gradually progress through the stages of maturity inside the follicles, in a process akin to a production line, Pal told Live Science.
"So, if you suppress the ovarian function, particularly with higher-dose oral contraceptives, you are slowing that production line," Pal told Live Science.
However, that doesn't mean these women's egg quality has permanently declined. Instead, hormonal birth control simply suspends women's egg maturation process in an earlier stage. Women who are pregnant see a similar decline in their AMH levels, which soon rebound after pregnancy, she said.
Birth control "just puts a brake on the whole process, and if you take the brake off, it takes a few months and the whole process is reinitiated," she said.
Once women stop taking the pill, they are usually able to conceive within a few months, she said.
The findings were presented today (July 1) at the European Society of Human Reproduction and Embryology conference.
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Tia is the managing editor and was previously a senior writer for Live Science. Her work has appeared in Scientific American, Wired.com and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.