Ectopic pregnancy happens when a fertilized egg implants outside the uterus, most often in a fallopian tube. It affects roughly 1% to 2% of all pregnancies in the United States. The causes come down to anything that slows or blocks the egg’s journey from the fallopian tube to the uterus, giving it time to implant in the wrong place.
How a Fertilized Egg Gets Stuck
After an egg is fertilized in the fallopian tube, three forces work together to push it toward the uterus: tiny hair-like structures called cilia that line the tube and sweep the egg forward, rhythmic muscle contractions in the tube wall, and a flow of fluid inside the tube. The cilia do most of the heavy lifting. Animal studies show that even when muscle contractions are completely shut down, the cilia alone can move an egg through the wider part of the tube.
At two key bottleneck points along the tube, the muscles stay tightly contracted and only relax once the uterine lining is ready to receive an embryo. Progesterone rising in the second half of the menstrual cycle triggers that relaxation. If any part of this system is damaged or mistimed, the egg may stall in the tube long enough to start implanting there instead of in the uterus.
Pelvic Infections and Tubal Scarring
Infections in the reproductive tract are the single most common underlying cause. Pelvic inflammatory disease (PID), usually caused by sexually transmitted bacteria, roughly doubles the risk of ectopic pregnancy. The infection triggers inflammation that creates scar tissue and adhesions inside and around the fallopian tubes. Even after the infection clears, the structural damage remains.
Chlamydia is particularly destructive. It attacks the cells lining the fallopian tube directly, stripping away the cilia and breaking down the connections between cells. Over time, this leads to permanent scarring. Chlamydia also disrupts the tube’s smooth muscle, which can cause fluid to build up and block the tube entirely. Because chlamydia often produces mild or no symptoms, many people don’t know they’ve had it until they experience fertility problems or an ectopic pregnancy.
Smoking and Tubal Function
Cigarette smoking is one of the strongest modifiable risk factors. Smokers face roughly 1.7 to 3.9 times the risk of ectopic pregnancy compared to nonsmokers, and women who smoke more than 20 cigarettes a day have a fourfold increase in risk.
Nicotine doesn’t just affect the lungs. It alters how the fallopian tubes contract and reduces blood flow to them. Research has identified a specific mechanism: cotinine, an active byproduct of nicotine, changes the levels of a protein that regulates smooth muscle movement in the tube. This disrupts the tube’s ability to propel the egg forward at the right pace. The damage is dose-dependent, meaning heavier smoking carries a higher risk, but even light smoking raises it.
Previous Tubal Surgery
Any surgery on or near the fallopian tubes increases ectopic risk. This includes surgery to reverse a tubal ligation, procedures to clear a blocked tube, or removal of a previous ectopic pregnancy. Between 3% and 20% of women who undergo tubal reconstructive surgery will have an ectopic pregnancy afterward. The wide range depends on how much damage was present before surgery and the specific technique used.
Even surgery to remove one fallopian tube (salpingectomy) after a prior ectopic pregnancy doesn’t eliminate future risk, because the remaining tube may also have underlying damage that wasn’t visible.
Endometriosis
Severe endometriosis, where tissue similar to the uterine lining grows in and around the fallopian tubes, can distort the tubes and create scar tissue that traps a fertilized egg. There’s also evidence that even milder endometriosis may interfere with the cilia’s beating rhythm or disrupt normal muscle contractions in the tube, though this link is less firmly established than for infections or smoking.
IVF and Assisted Reproduction
This one surprises many people: IVF increases ectopic risk even though the embryo is placed directly into the uterus. Women undergoing IVF have roughly two to three times the ectopic rate seen in natural conception, and the overall rate in IVF pregnancies ranges from about 2% to nearly 9%. For women with a history of tubal problems, the rate can reach 11%.
Several factors explain this. The hormonal stimulation used during IVF cycles can impair tubal function and alter the uterine lining’s receptivity, making it harder for the embryo to implant where it should. The uterus can also generate abnormal contractions after embryo transfer that push the embryo backward into a fallopian tube. Even the physical technique of transfer matters: the volume of fluid used and the pressure applied can force an embryo through the tubal opening. Women with pre-existing tubal disease face a compounded risk from both the tubal damage itself and the hormonal effects of the IVF cycle.
Other Risk Factors
An intrauterine device (IUD) does not cause ectopic pregnancy, but if a pregnancy does occur while an IUD is in place, it’s more likely to be ectopic. This is because the IUD is highly effective at preventing implantation in the uterus but has less effect on implantation elsewhere.
Age also plays a role. Women over 35 have a higher incidence, likely due to cumulative changes in tubal function over time. And a prior ectopic pregnancy is one of the strongest predictors of having another one: recurrence rates range from 10% to 27%. Women who have had two prior ectopic pregnancies face a tenfold increase in the risk of a third, and their chance of carrying a naturally conceived pregnancy to term drops to around 4%.
Recognizing the Symptoms
Symptoms typically develop between weeks 4 and 12 of pregnancy. Early signs often mimic a normal pregnancy or an early miscarriage: a missed period, mild cramping, and light vaginal bleeding. As the ectopic pregnancy grows, pain tends to concentrate on one side of the lower abdomen and become sharper.
In a normal early pregnancy, levels of the hormone hCG roughly double every two days. With an ectopic pregnancy, hCG still rises but more slowly, often failing to increase by even 50% over 48 hours. Doctors use this pattern alongside ultrasound to make the diagnosis. With transvaginal ultrasound, a normal pregnancy in the uterus is typically visible once hCG reaches 1,000 to 2,000 IU/L. If levels are above that threshold and no pregnancy is seen in the uterus, an ectopic location becomes the leading concern.
If the ectopic pregnancy ruptures the fallopian tube, symptoms escalate quickly: sudden, severe abdominal pain, dizziness, shoulder tip pain (caused by internal bleeding irritating the diaphragm), and sometimes fainting. A rupture is a medical emergency requiring immediate treatment.