For many uterus owners using birth control, the end goal is the same: to prevent conception and pregnancy. But just because the final destination is the same doesn’t mean that all reversible birth control methods (i.e., the methods that don’t involve surgical removal of reproductive organs) take the same path to get there.
Over the last 60-ish years, scientists have harnessed our knowledge of the different steps necessary to establish a pregnancy to create powerful, effective methods of birth control — not all of which work in exactly the same way.
In this post, we’ll go over the similarities and differences in how different popular birth control methods work to prevent pregnancy. We will also specifically explain how birth control impacts whether or not you ovulate.
Conception 101: What needs to happen?
To understand how different birth control methods work, we need to first understand the chain of events that is required for conception to occur. Though there are seemingly infinite things that need to go right for a pregnancy to be established, there are three main obstacles that a sperm and egg have to overcome (and these are all non-negotiable!):
- Ovulation: A developed egg needs to be ovulated (i.e., released into the fallopian tubes), after which it’ll hang around to see whether it can meet up with a spermy suitor. An egg can be fertilized between 12 and 24 hours after its release, and if fertilization by sperm doesn’t happen by the end of that window, RIP egg.
- Sperm meets egg: Sperm need to get past the cervix, to then continue making their way through the uterus and eventually to the fallopian tube and egg. If an egg isn’t around when sperm get there, that’s okay too — sperm can survive up to 5 days in the female reproductive tract.
- Implantation: The sperm + egg combo needs to nestle into a thick and blood vessel-rich uterine lining, which supports the development of the embryo, in a process called implantation. Implantation usually takes place 8-10 days after fertilization.
Substantial snags in any single step, or in multiple steps, make conception a whole lot harder. Think about it like an obstacle course: To get from the start to the finish line (where the finish line is getting pregnant), you need to get past each and every hurdle.
Birth control works by making several of these obstacles almost impossible to overcome.
How does your birth control method work?
Before we start nerding out about the science behind the pregnancy-blocking obstacles different birth control methods introduce, here’s a quick summary of how your current birth control method may work.
Table note: Typical use failure rates from the CDC. Source for additional data points: Mirena, Liletta, Kyleena, Skyla, Copper IUD. Copper IUDs are a bit of an outlier in that copper ions released from the implant create a local, non-dangerous inflammatory response that makes the uterus toxic for sperm (think of it as sperm-neutralizing).
- Combined oral contraceptive pills (COC): Do you ovulate on combination birth control pills? Short answer: No. The duo of synthetic progestins and estrogens in combination pills work on the brain to prevent the release of hormones needed for follicular development and ovulation (LH and FSH), thereby blocking ovulation. They also thicken the cervical mucus to make it harder for sperm to get through, and prevent thickening of the uterine lining. (E.g., Yasmin, Ortho Tri-Cyclen, and Loestrin FE.)
- Minipill: (aka progestin-only pill) The dose of progestins in the minipill is not high enough to prevent ovulation for all users. But the minipill exerts conception-blocking effects for all users through thickening the cervical mucus, and preventing thickening of the uterine lining. (E.g., Camila and Ortho Micronor.)
- Ring and patch: You can think of the ring and patch as the same as combination pills, with the only difference being how the hormones get into the body and how often you have to think about taking them. The progestins and estrogens in these methods work on the brain to prevent the release of LH and FSH (which means no ovulation), and also thicken cervical mucus and prevent the thickening of the uterine lining. (There is one monthly ring, NuvaRing, one year-long ring, Annovera, and one monthly patch, Xulane, approved for use in the US.)
- Implant and injectable: These work like the ring and the patch (despite not containing estrogens), by preventing ovulation, thickening cervical mucus, and preventing the thickening of the uterine lining. (The only implant approved for US use currently is Nexplanon, and the best-known injectable is Depo-Provera.)
- Hormonal IUD: The progestin in hormonal IUDs primarily thickens the cervical mucus, making it near impossible for sperm to get through, and secondarily, prevents the thickening of the uterine lining. The extent to which hormonal IUDs block ovulation depends on the hormone dose but overall, ovulation stops in just a small percentage of cases. (Also: All hormonal IUDs are considered “low dose,” but Kyleena and Skyla are considered *very* low dose relative to Mirena and Liletta, which have a longer uterine shelf life.)
- Copper intrauterine device (IUD): The copper ions released into the uterus by the copper IUD have a powerful sperm-neutralizing effect. Ovulation is not impacted by copper IUD use. (The only copper IUD approved for use in the US is Paragard.)
Birth control-induced obstacle #1: Preventing ovulation
The majority of reversible hormonal birth control methods work by preventing ovulation. To understand how this happens, we turn back to our understanding of the basic physiology that underlies ovarian follicular development and ovulation.
As the name implies, FSH facilitates the development and maturation of follicles that house eggs, and LH is the hormone that coaxes developed eggs to be released from follicles and into the fallopian tubes (aka ovulation), where it can potentially meet up with sperm. Some estradiol and progesterone, produced by both developing and ruptured follicles, make their way to the brain where they tell it to chill on its production of both FSH and LH.
In methods of birth control that release synthetic hormones similar to estradiol and progesterone into your bloodstream, we see a similar thing happen — levels of these hormones are consistently high enough to inhibit the production and release of FSH and FSH. And if there’s no follicular development, and nothing to stimulate the release of eggs from follicles, there’s no ovulation.
Caveat: Not all effective methods of hormonal birth control always prevent ovulation (we’re looking at you, minipill and hormonal IUDs). Though both of these methods contain progestins which do make it to the brain and inhibit some of the production and release of FSH and LH, the progestin doses are low enough that in some cases, FSH and LH levels aren’t suppressed enough to consistently inhibit follicular development and ovulation.
This is especially true in the case of hormonal IUDs, where most of the progestin released acts on physiological structures in its immediate vicinity, and only small amounts make their way into the bloodstream and to the brain. Why ovulation is impaired in only some users or only in some cycles is still a mystery, and we aren’t yet able to predict who will or will not continue ovulating when using the minipill or hormonal IUD.
Birth control-induced obstacle #2: Thickening cervical mucus
Cervical mucus closely tracks hormone levels — more specifically, estradiol and progesterone levels — and this tracking is so close that many people with ovaries monitor their cervical mucus to estimate whether they’re in their fertile window (aka when the chances for conception are highest).
High estradiol and low progesterone, which is characteristic of the fertile window, brings about a special type of cervical mucus (slippery, clear, stretchy “egg white” cervical mucus) that actually helps sperm get past the cervix and to the egg (assuming one was released during ovulation), thereby making conception more likely.
Any birth control that raises progesterone or lowers estradiol will make it really tricky (and by that, we mean almost impossible) for sperm to make it past the cervix and any closer to an egg. Bottom line? This means that if you are on a type of birth control that thickens your cervical mucus, you should never be seeing that slippery, clear, stretchy “egg white” mucus.
Birth control-induced obstacle #3: Preventing endometrial thickening
If for some reason something goes awry and some sperm make it past the cervix and to the uterus, all reversible birth control methods have your back with a powerful backup plan. For a fertilized egg to develop and grow, it needs to set up shop in the uterine lining, which later gives rise to structures like the placenta which are *crucial* for its development.
In typical menstrual cycles without hormonal birth control, progesterone and estradiol thicken the uterine lining and make it more receptive to, and nourishing for, a fertilized egg. Hormonal birth control methods work by controlling the amount of progesterone (and sometimes estradiol) — making the uterine lining too thin and undeveloped for successful implantation and pregnancy (think of this like prolonging the state the uterine lining is in right after you finish your period).
Because birth control methods are extremely effective at preventing an egg from being fertilized in the first place (via obstacles #1 and #2 as discussed above), this endometrial-thinning-backup-plan doesn’t need to get used all that often. But even if somehow an egg does get fertilized while you’re on birth control, this alone doesn’t add up to a pregnancy; the American College of Obstetricians and Gynecologists and other reproductive health organizations have defined a pregnancy as starting once a fertilized egg implants into the uterine lining.
The special case of copper IUDs: In the case of copper IUDs, copper ions released from the implant create a local, non-dangerous inflammatory response that makes the uterus toxic for sperm (think of it as sperm-neutralizing). The inflammatory response caused by the presence of the copper ions makes it harder for sperm to make their way around, and also makes the uterine lining less hospitable to a hopeful fertilized egg. So while the copper IUD doesn’t prevent the thickening of the uterine lining (which is why most copper IUD users will continue to get regular periods!), it does induce other changes in the uterus that make it a not-conception-friendly place to be.
To ovulate or not to ovulate? A personal preference
You might be wondering: If most reversible birth control methods change cervical mucus and the uterine lining, what are the pros/cons to choosing a birth control method that also suppresses ovulation? After all, the birth control methods that always suppress ovulation are not more effective than those that don’t, when you look at typical use success/failure rates.
This choice ultimately comes down to what your personal preference is. Suppressing ovulation by birth control is neither “bad” or “good.” What we mean by that: There’s no scientific evidence that not ovulating now will make it harder to ovulate later — but it also won’t “save” eggs and leave you more later in life. Though usually one egg is released from one follicle during ovulation, somewhere between 800 and 1,000 follicles naturally die off each month (regardless of whether you are on birth control or ovulating) through a natural process called “apoptosis” — which makes the one egg you’d potentially “save” each month while being on ovulation-suppressing birth control seem pretty inconsequential in comparison.
Some people might be more comfortable with birth control methods that have less or no impact on their natural production of reproductive hormones. They may like knowing that they’re still ovulating even if they’re not trying to conceive right now, and can use ovulation tests that measure LH for this reason. Others seeking to avoid conception may find greater comfort in knowing that ovulation is shut down altogether, making methods that reliably suppress ovulation (if taken as prescribed!) the way to go.
Wrapping things up
If we think of conceiving as an obstacle course, modern birth control methods are considered the ultimate obstacle-creating masters. They work by some combo of suppressing ovulation, making cervical mucus unfriendly to sperm, and blocking the development of an implantation-friendly uterine lining — the best defense is a good offense.
Because ovulation isn’t always suppressed in users of the minipill, copper IUD, and hormonal IUD, it’s still possible to monitor your LH and ovulation if you use one of these birth control methods (after all, keeping tabs on ovulation isn’t just for people trying to get pregnant right now!).
One option we’re admittedly fond of for keeping tabs on your ovulation? Modern Fertility’s Ovulation Test and free app. It’s designed for people with ovaries interested in understanding what’s going on in their bodies ovulation-wise, whether they’re trying for kids right now or not. Our test identifies your LH at low, high, or peak levels. Tracking your LH daily gives you more insight into your fertile window and ovulation patterns than you’d get with standard ovulation tests that give positive/negative results.
This article was reviewed by Dr. Jennifer Conti, MD, MS, MSc. Dr. Conti is an OB-GYN and serves as an adjunct clinical assistant professor at Stanford University School of Medicine.