Plan B is not harmful if you are already pregnant. Multiple studies comparing pregnant women who were exposed to levonorgestrel (the hormone in Plan B) with those who were not have found no increased risk of miscarriage, birth defects, or complications. If you took Plan B and later discovered you were already pregnant, the existing evidence is reassuring.
How Plan B Works (and Why It Can’t End a Pregnancy)
Plan B works primarily by delaying or preventing ovulation, the release of an egg from the ovary. If ovulation hasn’t happened yet, sperm have nothing to fertilize, and pregnancy is prevented. This is the pill’s main mechanism.
Crucially, Plan B does not end a pregnancy that has already started. Once a fertilized egg has implanted in the uterus, levonorgestrel has no effect on it. This is the key distinction between emergency contraception and the abortion pill (mifepristone), which belongs to a completely different class of drugs called antiprogestins. Mifepristone blocks the hormone progesterone, which is needed to sustain a pregnancy. Plan B does not do this. The two medications work in fundamentally different ways, despite frequent confusion between them.
What the Research Shows About Fetal Safety
Several studies have directly tracked pregnancies where women took Plan B around the time of conception or in very early pregnancy, then compared outcomes with pregnancies that had no exposure.
A prospective cohort study matching 332 exposed women to 332 unexposed women found no differences in first-trimester miscarriages or congenital malformations, whether detected on ultrasound or at delivery. No maternal complications were reported in either group, and newborns showed no differences in weight, length, or risk of malformations.
A separate prospective cohort study tracked pregnancy outcomes after levonorgestrel emergency contraception failure. Miscarriage rates were statistically identical between the exposed and unexposed groups, and four malformations were found in each group. Average birthweight was actually slightly higher in the exposed group (3,416 grams versus 3,345 grams). The researchers concluded there was no association between levonorgestrel exposure and the risk of major congenital malformations, pregnancy complications, or any other adverse outcomes.
Long-Term Child Development
One concern that goes beyond pregnancy outcomes is whether exposure to Plan B during conception might affect a child’s development later on. A follow-up study tracked 195 children who were exposed to levonorgestrel during their mothers’ conception cycle and compared them to 214 children without exposure. The rate of congenital malformations was similar: 2.1% in the exposed group versus 1.4% in the control group, a difference that was not statistically significant. More importantly, the study found no effect on physical growth or mental development in the children. In practical terms, kids whose mothers took Plan B around the time they conceived developed normally.
What Medical Guidelines Say
Professional guidelines reflect this safety data clearly. The Society of Family Planning states that for patients who take levonorgestrel emergency contraception while unknowingly pregnant, or who conceive after the pill fails, “there is no concern for associated pregnancy-related morbidity.” Meta-analyses have not found a link between embryonic exposure to contraceptive hormones and fetal malformations of any kind.
The guidelines go a step further: they recommend against withholding or delaying emergency contraception for a pregnancy test. That recommendation exists precisely because the safety data is strong enough that a possible existing pregnancy is not a reason to avoid taking Plan B. Pregnancy testing beforehand is considered unnecessary unless you want to know your pregnancy status for other reasons.
A Note on Ectopic Pregnancy
One complication that occasionally comes up in connection with Plan B is ectopic pregnancy, where a fertilized egg implants outside the uterus (usually in a fallopian tube). A case-control study from Kenya found that women with ectopic pregnancies were more likely to have a history of using levonorgestrel emergency contraception. However, the researchers attributed this to contraception failure rather than a direct effect of the drug itself. When Plan B fails to prevent pregnancy, the pregnancies that do occur may include ectopic cases at a rate that doesn’t exceed what’s seen in the general population. Systematic reviews have placed the ectopic pregnancy rate following Plan B failure at about 1.6%, which falls within the normal range of 0.8% to 2%.
This doesn’t mean Plan B causes ectopic pregnancy. It means that if you take Plan B and still become pregnant, there’s the same small chance of an ectopic pregnancy that exists with any conception. If you experience sharp abdominal pain, unusual bleeding, or dizziness after a positive pregnancy test, those are signs of ectopic pregnancy regardless of whether you’ve taken emergency contraception.
What About Ella (Ulipristal Acetate)?
Ella is a different type of emergency contraception that belongs to the antiprogestin drug class. The safety data for ulipristal acetate in pregnancy is more limited than for Plan B, but still generally reassuring. The miscarriage rate among pregnancies exposed to Ella is about 13.8%, which is well within the 20% miscarriage rate seen in the general population. The ectopic pregnancy rate (1.1%) is also similar to baseline levels. No pregnancy or delivery complications have been reported in association with Ella use, aside from a single case of optic nerve atrophy that an independent safety board determined was not caused by the drug.
That said, because Ella is pharmacologically closer to mifepristone than Plan B is, the data is worth watching more closely. If you took Ella and are now pregnant, the existing evidence doesn’t point to harm, but the body of research is smaller.