Between 2017 and 2019, over 10 million U.S. women of reproductive age were on the Pill. Over 7.5 million were using an IUD or arm implant, according to the Centers for Disease Control and Prevention (CDC). Many of these women started taking hormonal birth control much earlier than their mothers and grandmothers did, and they’re staying on it for years—even decades— on end. In the context of a major uptick in the number of women struggling to get pregnant (the CDC estimates that 1 in 5 women trying to conceive for the first time are unable to get pregnant despite trying for a year), some may wonder: could the two be connected? Could hormonal birth control— be it the Pill, the shot, the patch, or the vaginal ring— cause or contribute to infertility?
Medical sources often claim that birth control can’t cause infertility
Some medical sources like this research study reassure women “Contraceptive use regardless of its duration and type does not have a negative effect on the ability of women to conceive following termination of use and it doesn’t significantly delay fertility.” And popular health websites like Healthline (which cites the research study above) insist, “hormonal contraceptives don’t cause infertility, no matter which method you use or how long you’ve been using it.”
But there’s reason to believe this answer may be overly simplistic. Conception requires sperm, an egg, and good quality cervical mucus. Healthy ovaries are necessary both for the release of mature eggs (via ovulation) and for production of stretchy cervical mucus capable of guiding sperm to an egg.
Research suggests that hormonal birth control can both negatively impact cervical mucus quality and speed up the normal process of ovarian aging by decreasing egg quality and quantity. Indirectly, then, birth control use—especially long-term—could possibly contribute to infertility.
What is ovarian reserve and how is it connected to fertility?
Ovarian reserve is an indirect indicator of fertility. Ovarian reserve is the ovaries’ ability to produce mature eggs that could potentially be fertilized. You may have heard that a woman is born with all the eggs she’ll ever make in her life. This is true. All the eggs your body has ever made were created while you were still in utero. This means that the egg that joins with a sperm to become your baby was produced while you were still growing inside your mother’s (and your baby’s grandmother’s) womb!
But the eggs in your ovaries are immature and each one is contained within a microscopic, dormant follicle (picture a follicle like a dormant incubator capable of maturing a single egg). If you’re not on birth control, your body naturally selects one follicle to mature each month, and follicle stimulating hormone (FSH) kickstarts this process. Ovulation is the monthly release process of (usually) just one mature egg from a follicle. If this mature egg is fertilized by sperm, new human life results.
The amount of eggs capable of maturing naturally decreases as a woman ages, and when the egg count hits zero, the woman is in menopause. The ovarian reserve, then, is expected to decrease over time. We generally see evidence of this in decreased rates of fertility, which can start as early as a woman’s mid- to late-30s.
How do we measure ovarian reserve?
There are three main ways to measure ovarian reserve, and two of them require an ultrasound (or a series of ultrasounds to compare results over a period of time) [1]. The three markers are anti-Mullerian hormone (AMH), antral follicle count (AFC), and ovary volume. As discussed above, an egg matures inside a follicle, and follicles produce AMH. AMH levels, then, give an idea of how many eggs-inside-follicles remain in the ovaries. AFC is a count of how many immature follicles a woman has, and together with ovary volume (size), can be measured by ultrasound.
One other indicator of ovarian reserve is a blood draw to check a follicle-stimulating hormone (FSH) level. More on this below.
How do we know the ovaries are or might be “aging” prematurely?
A woman’s ovaries could prematurely age— meaning that her ovarian reserve is low compared to what’s expected based on her age— in one or both of two ways. Prematurely aged ovaries mean that egg quality and/or quantity may be lower than expected for her age range, or cervical mucus quality is be poor.
Signs of low egg quantity include a small ovary, low AFC on ultrasound, or a high FSH level in the blood (or some combination of the three) [2]. Elevated FSH levels occur when smaller amounts of FSH released by the pituitary gland in the brain are no longer adequate enough to stimulate a follicle to develop. The body recognizes that higher and higher amounts are needed to achieve the desired effect of follicle maturation, and releases it accordingly.
How much might hormonal birth control prematurely age the ovaries and impair fertility?
Hormonal birth control overrides the female body’s natural menstrual cycle and stops it from happening altogether. What effect might this disruption of normal, cyclical, hormonal influence on the ovaries have? Even though birth control stops your cycle, it doesn’t slow down or stop the normal egg loss that occurs to aging. And even though medical websites like Cleveland Clinic don’t mention it, studies show that HBC use could actively decrease ovarian reserve.
According to a 2015 study, “[Oral contraception] has a major impact on the ovarian volume, and a moderate impact on AFC and AMH” [3]. Ovarian volume was reduced by a full 50% in women who used the Pill compared to women who weren’t on hormonal birth control (HBC).
In a 2020 study, the Pill (combined synthetic estrogen and progestin), progestin-only Pill, and the Levonorgestrel-IUD all decreased AFC and AMH levels. Users of the Pill had the biggest decreases in AFC (31.3% lower counts than non-HBC users), and AMH (31.1% lower levels than non-HBC users).
Additionally, in a 2022 meta-analysis, AMH levels decreased in oral contraceptive users as early as three months after starting the Pill.
Finally, HBC use may also lead to cervical mucus changes consistent with more infertile-type mucus, according to this 2023 study [1].
The bottom line on hormonal birth control use and infertility
It’s true that conception only requires adequate cervical mucus for one sperm to reach one egg. But we know that decreased ovarian reserve translates to decreased fertility, and we know that hormonal birth control use can decrease ovarian reserve according to multiple parameters. Once decreased, ovarian reserve cannot be increased. Once the eggs are gone, they’re gone. Women who take HBC deserve to know about the possibility that their current contraceptive choice may impact their future childbearing. So does hormonal birth control “cause” infertility? Not exactly. But it certainly could contribute by prematurely aging the ovaries.
References:
[1] Segarra, Ignacio et al. “Women’s health, hormonal balance, and personal autonomy.” Frontiers in medicine vol. 10 1167504. 30 Jun. 2023, doi:10.3389/fmed.2023.1167504 [2] Kelsey TW, Wallace WH. Ovarian volume correlates strongly with the number of nongrowing follicles in the human ovary. Obstet Gynecol Int. 2012;2012:305025. doi: 10.1155/2012/305025. Epub 2012 Feb 12. PMID: 22496698; PMCID: PMC3306948. [3] Birch Petersen, K et al. “Ovarian reserve assessment in users of oral contraception seeking fertility advice on their reproductive lifespan.” Human reproduction (Oxford, England) vol. 30,10 (2015): 2364-75. doi:10.1093/humrep/dev197 [4] Landersoe, Selma Kloeve et al. “Ovarian reserve markers in women using various hormonal contraceptives.” The European journal of contraception & reproductive health care : the official journal of the European Society of Contraception vol. 25,1 (2020): 65-71. doi:10.1080/13625187.2019.1702158[5] Yin, Wei-Wei et al. “The effect of medication on serum anti-müllerian hormone (AMH) levels in women of reproductive age: a meta-analysis.” BMC endocrine disorders vol. 22,1 158. 14 Jun. 2022, doi:10.1186/s12902-022-01065-9
