Disclaimer: I'm not a medical professional. I share my own experiences and cite my research at the end of each article on this site. Please consult a medical professional for your own diagnosis and treatment.
Whether you're on the pill, the patch, the ring, the shot, or a hormonal IUD, you have one thing in common: you're replacing your body's natural supply or progesterone (and sometimes) estrogen with synthetic versions of these hormones.
Hormonal birth controls (HBCs) use two key mechanisms to eliminate your chance of pregnancy:
Synthetic estradiols work to keep your body from releasing a mature ova (ovulation) - HBC mimics the second phase of the menstrual cycle to convince your body it's already ovulated.
Synthetic progestins work to thicken your cervical mucus, making it impossible for sperm to swim freely towards the fallopian tubes, and/or to thin the endometrial lining, making it impossible for a fertilized egg to implant**.
As you can see, there are a lot of "fail-safes" included in these tiny hormonal packages. For this reason, it's reeeeally unlikely that a diligent HBC-taker will get pregnant. Only about 1/1000 people will get unintentionally pregnant if using these methods appropriately*.
However, synthetic estradiols and progestins aren't all sunshine and rainbows. As you likely know and may have experienced, birth controls often lead to tons of unwanted side effects. Why?
Let's first consider the obvious:
By replacing your body's natural production of estrogen and progesterone with synthetic versions, you dramatically suppress your natural estrogen and testosterone levels to the effect that the natural hormone profile of someone taking HBC will look more like that of someone in menopause.
It's critically important to understand that your naturally occurring estrogen and progesterone are not just two hormones that regulate when you bleed each month, but rather these are essential elements of your overarching bodily functions. As (my alma mater) Johns Hopkins puts it, even just estrogen plays a myriad of non-sex-related roles in the body: "In addition to regulating the menstrual cycle, estrogen affects the reproductive tract, the urinary tract, the heart and blood vessels, bones, breasts, skin, hair, mucous membranes, pelvic muscles, and the brain. Secondary sexual characteristics, such as pubic and armpit hair, also start to grow when estrogen levels rise. Many organ systems, including the musculoskeletal and cardiovascular systems, and the brain are affected by estrogen." (Source)
As one renowned OB-GYN described the impact of this reduction in natural hormone production in otherwise healthy women,
"The havoc occurs slowly, but inexorably...Studies show birth control pills reduce the development of strong bones, negatively impact gut and immune health, damage vaginal and bladder health, and impact brains."
(Source)
In this very real sense, some of the side effects you may experience on the pill may be symptoms of natural hormonal absence. In my own example, I experienced UTIs and yeast infections regularly on the combination birth control pill. I now recognize that this penchant towards infection was likely caused by a disruption of my vaginal pH, due to lack of estrogen. Longer-term side effects from decreased levels of natural estrogen in the body would include a higher likelihood of osteoporosis*.
So, if you don't have your own natural hormones anymore, what are you left with?
Synthetic estradiol and progestins are still predominantly made of a strong androgenic (male) hormone, nandrolone, which is more stable than natural estrogen and progesterone, themselves.*
While people have long complained of the strange masculinizing physical side effects of HBCs, such as acne, random facial hair growth, or male pattern hair loss, etc., scientists are now studying and documenting the diverse cognitive impacts of HBCs on female brains.
About 83% of pill-takers continue taking those derived from androgenic hormones (they're typically more generic, cheaper prescriptions) and the brains of these pill-takers show enlarged areas of the brain aligned to areas that would be enlarged in male brains, too.
Studies have shown these pill-takers respond less to emotions and remember stories more similarly to men than non-pill-taking women, they experience a reduction in articulateness and an altered ability to organize and rotate shapes.*
Among the other 17% of pill-takers, their pill is formulated with an anti-androgen included, and this is aimed at combatting some of these masculinizing effects. Still, however, among these newer-formulated pill-takers, two areas of the brain appear enlarged: the fusiform face area and the parahippocampal place area, which help people recognize faces and places respectively*.
Clearly, synthetic hormones work to change your brain and the body.
Finally, I want to talk about a more mechanical element of all of this. Regardless of the difference between synthetic and natural hormones, which we have discussed up until this point, HBC works by manipulating and eliminating the follicular phase of your cycle to prevent ovulation:
The very hormonal mechanism that stops you from ovulating puts people on HBCs into a sort of permanent luteal phase- the second half of your typical menstrual cycle, that occurs after ovulation and before menses.
I've said it before and I'll say it again - when you're in a semi-permanent luteal phase, you're going to likely experience all of the luteal phase symptoms you would typically experience in a natural cycle more frequently, intensely, and in some cases, ceaselessly. Recently, for example, a reader asked me why she was experiencing constant breast tenderness and pain after 6-ish months on progestin-only pill. My answer was essentially, "Well, high-levels of progesterone make your boobs hurt." After six months of consistently ingesting progestin, her body was just responding accordingly.
Another question I see all the time is "Why am I gaining weight on birth control?" and while doctors and scientists love to talk about the fact that there is no direct causation determined between birth control and weight gain, there's certainly a scientific explanation for weight gain when consistently taking synthetic progestins. Progesterone is known to increase appetite and cravings during the luteal phase. Naturally, if you extend this phase beyond its natural 12-14 days to a permanent every day, those cravings will add up. And beyond your cravings and appetite, progesterone also increases fatigue, making you less likely to want to burn off that slice of cake on an elliptical.
All of this is to say that the mechanisms and not just the formulation of HBCs can also create behavioral and symptomatic feedback loops for us.
End Note:
If you were hoping to read about a specific side effect I didn't mention in this article, I'm here for you! I have written about my own experiences with:
I've also shared a reader story about a blood clot scare, here.
*Resources:
This is Your Brain on Birth Control, Sarah Hill, PhD
**Note: Many progestin-only contraceptives exist, including the progestin-only pill (the mini-pill), the shot, the implant, and hormonal IUDs. These contain no synthetic estrogen.
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