Pregnancy can be stopped at several points, from preventing an egg from being released to ending an early pregnancy that has already begun. The method depends entirely on timing: whether you’re trying to prevent conception before it happens, block a pregnancy in the days after unprotected sex, or terminate an existing pregnancy. Here’s how each approach works.
How Hormonal Birth Control Prevents Pregnancy
Hormonal contraceptives work through two main mechanisms. First, they suppress ovulation, meaning your ovaries don’t release an egg. No egg, no pregnancy. Second, they thicken cervical mucus so sperm can’t pass through to reach an egg even if one were released. Research on combined oral contraceptives confirms that both of these effects work together to provide protection.
The catch is consistency. Under perfect use, the pill, patch, and similar methods have a failure rate of just 0.3%. But in real life, with missed doses and late refills, about 9 out of 100 women using the pill or patch will get pregnant in a given year. The injectable shot performs slightly better at 6% with typical use, partly because you only need it once every three months instead of remembering a daily pill.
One significant change in access: the FDA has approved the first over-the-counter daily birth control pill (sold as Opill), a progestin-only tablet available without a prescription at pharmacies, grocery stores, and online. This removes one of the biggest barriers to consistent use.
Emergency Contraception After Unprotected Sex
If you’ve had unprotected sex or a contraceptive failure, emergency contraception can still prevent pregnancy, but the window is limited and the options differ in important ways.
The most common emergency pill (containing levonorgestrel, sold as Plan B and generics) works by delaying or blocking ovulation. It’s effective when taken before ovulation occurs, but once your body is on the verge of releasing an egg, this pill loses its ability to stop the process. It should be taken as soon as possible within 72 hours of unprotected sex.
A second pill option (containing ulipristal acetate, sold as ella) has a direct inhibitory effect on the follicle that releases the egg. This means it can still work even when taken shortly before ovulation, a window where levonorgestrel is no longer effective. It’s approved for use up to 120 hours (5 days) after unprotected sex.
Body weight matters for both pills. Levonorgestrel becomes significantly less effective as BMI increases, with obese women facing roughly four times the risk of pregnancy compared to women at a normal BMI. Ulipristal shows a similar trend, though the data is less definitive. If weight is a concern, a copper IUD is the strongest option.
The Copper IUD as Emergency Contraception
The copper IUD is the most effective form of emergency contraception available. Across more than 8,500 documented emergency insertions, only 8 pregnancies occurred, a failure rate of about 0.09%. That’s over 99% effective, regardless of body weight, and it can be placed up to 5 days after unprotected sex.
Unlike the pills, which only delay ovulation, the copper IUD appears to have an additional mechanism: it changes the uterine environment in a way that prevents implantation. This likely explains why its emergency effectiveness is actually higher than its already-low failure rate during years of continuous use. Once placed, it also doubles as long-term contraception for up to 10 years.
What Naturally Stops a Pregnancy From Taking Hold
Even without any intervention, a large percentage of fertilized eggs never result in pregnancy. The body has its own screening process. Research suggests the uterine lining acts as a biological sensor: if an embryo carries serious genetic abnormalities or is otherwise non-viable, the uterine environment will not support implantation. The maternal immune system plays an active role here, balancing tolerance of a healthy embryo with the ability to reject one that won’t develop normally.
Age also plays a significant role. As women get older, the uterine lining becomes thinner, blood supply to the uterus declines, and hormonal regulation shifts. These changes make implantation progressively harder, which is one reason fertility drops with age even when ovulation still occurs. Conditions like uterine fibroids, polyps, and adhesions, all more common with age, further reduce the chances of a fertilized egg successfully implanting.
How Medication Abortion Ends an Early Pregnancy
Once a pregnancy is established, prevention is no longer the question. Medication abortion uses two drugs taken in sequence to end an early pregnancy, approved for use up to 10 weeks (70 days) of gestation.
The first medication blocks progesterone, the hormone that maintains the uterine lining and supports the pregnancy. Without progesterone, the lining thins and the pregnancy detaches. The second medication, taken 24 to 48 hours later, causes the uterus to contract and expel the pregnancy tissue. This typically involves several hours of cramping and heavy bleeding, similar to a miscarriage.
Evidence supports the use of this regimen beyond 10 weeks in some clinical settings, but the FDA-approved window remains at 70 days of gestation. Surgical options are available for pregnancies beyond that point.
Comparing Effectiveness at a Glance
- Copper IUD (emergency use): 99.9% effective, works up to 5 days after sex, unaffected by body weight
- Ulipristal acetate pill: effective up to 5 days after sex, works closer to ovulation than levonorgestrel, may be less effective at higher BMI
- Levonorgestrel pill: most effective within 24 hours, approved for use up to 72 hours, significantly less effective in obese women
- Daily birth control pill (typical use): 91% effective over a year, now available without a prescription
- Injectable contraception (typical use): 94% effective over a year
The gap between perfect and typical use for daily methods highlights a practical reality: the best contraceptive is the one you can use consistently. Long-acting methods like IUDs and implants remove the human error factor entirely, which is why they have the lowest failure rates of any reversible contraception.