Emergency contraception (EC) prevents pregnancy after unprotected sexual intercourse or contraceptive failure. The most common forms are levonorgestrel (Plan B) and ulipristal acetate (Ella). EC is not designed to terminate an existing pregnancy; it acts before a pregnancy is established. If a person is already pregnant, the medication will not end the pregnancy. Inadvertently taking EC during a very early, unrecognized pregnancy is not associated with harm to the developing fetus.
The Mechanism of Emergency Contraception
Emergency contraceptive pills work primarily by interfering with ovulation, the release of an egg from the ovary. The timing of administration relative to the menstrual cycle directly affects the medication’s effectiveness. Both levonorgestrel and ulipristal acetate disrupt the hormonal cascade that leads to the egg’s release.
Levonorgestrel, a synthetic progestin, functions by suppressing the pre-ovulatory surge of luteinizing hormone (LH), which signals the ovary to release an egg. By delaying or completely preventing this surge, the medication stops the egg from being released, thereby preventing fertilization. If the LH surge has already begun, the effectiveness of levonorgestrel is significantly reduced.
Ulipristal acetate, a selective progesterone receptor modulator, has a similar primary function but is more effective later in the pre-ovulatory window. It can postpone follicular rupture even after the LH surge has started, offering a longer window of effectiveness compared to levonorgestrel. Both medications may have secondary effects, such as altering the cervical mucus or tubal transport of sperm, but their main action is to prevent the union of sperm and egg.
A pregnancy is medically defined as beginning when a fertilized egg successfully implants into the wall of the uterus. Because the mechanism of EC is focused on preventing ovulation or fertilization, the medication loses its efficacy once implantation has occurred.
Safety Profile of EC Exposure During Early Pregnancy
Fetal safety when emergency contraception is taken during an unrecognized pregnancy is a common concern. Clinical studies have examined the outcomes of pregnancies that continued after inadvertent exposure to levonorgestrel. The consensus is that this exposure does not cause birth defects or increase the risk of poor pregnancy outcomes.
Research analyzing pregnancies following levonorgestrel use shows that rates of spontaneous abortion, stillbirth, and major congenital malformations are comparable between exposed and unexposed control groups. This suggests that the high doses of progestin are not teratogenic (do not cause developmental abnormalities). Exposure occurs during the very early stages of embryonic development, often before the person is aware they are pregnant.
For ulipristal acetate, the data pool is smaller than for levonorgestrel, but existing evidence does not suggest an increased risk of complications. Ulipristal acetate is considered low risk if taken inadvertently in early pregnancy. The drug’s mechanism does not support terminating an implanted pregnancy, and follow-up has not indicated a safety concern for the developing fetus.
These findings affirm that emergency contraception is not associated with an increased risk of complications, such as ectopic pregnancy. The well-established safety profile allows healthcare providers to reassure patients that if the medication fails to prevent pregnancy, the exposure itself is not a risk factor for the child’s development.
Differentiating Emergency Contraception from Abortion Medication
The difference between emergency contraception and medication used for abortion is significant, resting on their distinct mechanisms of action and the stage of pregnancy at which they are used. Emergency contraception prevents the initiation of a pregnancy by interfering with ovulation or fertilization. It is a form of prevention used before an embryo has implanted in the uterus.
Medication abortion, in contrast, is used to terminate an established pregnancy, meaning one that has already implanted. The process typically involves two different medications, mifepristone and misoprostol, which act on the uterine lining and muscle. These drugs are generally used several weeks after conception, once a pregnancy has been confirmed.
Mifepristone is the first medication taken and works by blocking the body’s progesterone receptors. Progesterone is the hormone necessary to maintain the uterine lining and sustain a pregnancy, so blocking its effect causes the pregnancy to detach. This action is fundamentally different from the hormonal surge suppression used by emergency contraception.
The second medication, misoprostol, causes the uterus to contract and expel the pregnancy tissue. Together, these two drugs actively interrupt the biological processes that support an implanted and growing pregnancy. This mechanism is in stark contrast to emergency contraception, which is rendered ineffective once implantation has occurred.
Emergency contraception is a pre-conception preventative measure, while medication abortion is a therapeutic intervention for an established pregnancy. Individuals who discover they are pregnant after taking emergency contraception should consult a healthcare provider for routine prenatal care and personalized advice, as the EC medication itself is not a concern for the continuing pregnancy.