A tubal pregnancy happens when a fertilized egg implants inside a fallopian tube instead of reaching the uterus. This accounts for the vast majority of ectopic pregnancies, which occur in roughly 2% of all reported pregnancies in the United States. The egg gets stuck because something has damaged, blocked, or altered the tube’s ability to move it along. Several specific conditions and risk factors make this more likely.
How a Fertilized Egg Gets Trapped
Under normal circumstances, the fallopian tube is an active transport system. Tiny hair-like structures called cilia line the inside of the tube, and smooth muscle in the tube wall contracts rhythmically to push the fertilized egg toward the uterus. When either of these mechanisms is compromised, the egg can slow down or stall, giving it time to implant in the tube wall rather than continuing its journey.
Research from PNAS has shown that cilia at the opening of the fallopian tube are essential for picking up the egg from the ovary in the first place. Farther along the tube, cilia help move the embryo but aren’t the only force at work. Smooth muscle contractions also push the egg forward. This means damage to the tube’s interior lining, its muscular wall, or both can interfere with transport and trap a fertilized egg at the wrong location.
Pelvic Infections and Sexually Transmitted Infections
The single most common cause of tubal damage is pelvic inflammatory disease (PID), an infection of the upper reproductive tract. Chlamydia and gonorrhea are the two infections most frequently responsible. These bacteria travel upward from the cervix into the uterus and fallopian tubes, triggering an intense inflammatory response. White blood cells flood the area to fight the infection, and the resulting inflammation can scar and distort the delicate inner lining of the tubes.
The damage doesn’t always cause obvious symptoms. Many chlamydia infections, in particular, produce no noticeable pain or discharge. A person can develop significant tubal scarring without ever knowing they had an infection. This is one reason tubal pregnancies sometimes seem to come out of nowhere. The scarring narrows the tube’s passageway, destroys cilia, and creates rough, adhesion-covered surfaces where a fertilized egg can get caught.
Smoking and Fallopian Tube Function
Smoking is one of the strongest modifiable risk factors for tubal pregnancy. Nicotine and its byproducts directly alter how the fallopian tubes work at a molecular level. Research published in PLOS ONE found that cotinine, an active breakdown product of nicotine, changes the expression of a protein that regulates smooth muscle contractions in the tube. This disrupts the rhythmic squeezing that normally propels the egg toward the uterus.
Smoking also shifts the balance between cell survival and cell death signals inside the fallopian tube. In smokers, genes that promote cell survival are turned up while those that trigger normal cell turnover are turned down. This imbalance can alter the tube’s internal environment in ways that make ectopic implantation more likely. The changes were measurable in tube tissue from smokers compared to nonsmokers, with roughly 1.5-fold differences in key gene activity.
Previous Surgery in the Abdomen or Pelvis
Any surgery near the fallopian tubes can create scar tissue that distorts or blocks them. This includes operations to remove ovarian cysts, treat fibroids, manage endometriosis, or address a previous ectopic pregnancy. Even procedures not directly involving the tubes, like appendix removal, can trigger adhesions that press on or kink the tubes from the outside.
Tubal surgery carries a particularly high risk. Operations to reverse a tubal ligation (sterilization) or to open a blocked tube leave the tube functional but scarred. The biggest risk after any tubal surgery is a future tubal pregnancy, because the interior may heal with irregularities that slow embryo transport. Adhesions can also form between the end of the tube and the ovary, making egg pickup less efficient and increasing the chance the fertilized egg lingers in the wrong place.
Structural Abnormalities of the Tubes
Some people have tubes that are misshapen from birth or develop structural changes over time. One well-documented condition, called salpingitis isthmica nodosa, involves small pouches (diverticula) forming in the narrow section of the tube closest to the uterus. In healthy, fertile women, this condition is found in fewer than 11% of cases. But it’s significantly more common in women who experience ectopic pregnancies or infertility, because those pouches can trap a fertilized egg before it reaches the uterus.
Endometriosis can also distort tubal anatomy. When endometrial tissue grows on or around the tubes, it causes chronic inflammation and scarring that narrows the passageway and damages the cilia lining.
Previous Ectopic Pregnancy
Having one tubal pregnancy substantially raises your risk of having another. The underlying condition that caused the first ectopic, whether scarring, structural abnormality, or chronic inflammation, is usually still present. Even if the affected tube is removed entirely, the remaining tube may share similar damage, especially if the original cause was a bilateral infection like chlamydia.
Fertility Treatments and IVF
Fertility treatments, including IVF, are associated with a higher rate of ectopic pregnancy. This might seem surprising since IVF places embryos directly into the uterus, bypassing the tubes entirely. But embryos placed in the uterus can migrate backward into a fallopian tube, particularly if the tube has structural abnormalities or altered muscular activity. Hormonal stimulation used during fertility treatment may also affect tubal contractions and embryo transport. Infertility itself is an independent risk factor, since many of the same conditions that reduce fertility (tubal damage, endometriosis, hormonal imbalances) also predispose to ectopic implantation.
Hormonal Factors and Contraceptive Use
Hormonal imbalances can play a role by affecting the timing and strength of tubal contractions. Progesterone, in particular, influences how the tube moves and how receptive its lining is to implantation. When progesterone levels are off, the egg may travel too slowly or the tubal lining may become more hospitable to implantation than it should be.
Certain contraceptive methods are worth noting here. Hormonal IUDs and progestin-only pills are highly effective at preventing pregnancy overall, but in the rare event that pregnancy does occur while using them, the proportion of those pregnancies that turn out to be ectopic is higher than normal. This doesn’t mean these contraceptives cause ectopic pregnancy. It means they’re better at preventing uterine pregnancies than tubal ones, which skews the ratio.
Recognizing the Warning Signs
A tubal pregnancy often starts out feeling like a normal early pregnancy, with a missed period and a positive test. Symptoms that distinguish it typically develop between weeks 4 and 12. Pain on one side of the lower abdomen is the most common early sign, sometimes accompanied by light vaginal bleeding or spotting that looks different from a normal period.
If a tubal pregnancy progresses without treatment, the growing embryo can rupture the tube. This is a medical emergency. The bleeding is primarily internal, not vaginal, which means you can lose a dangerous amount of blood without visible external bleeding. One distinctive warning sign of internal bleeding is sudden shoulder tip pain, which occurs because blood pooling in the abdomen irritates the diaphragm. The diaphragm shares nerve pathways with the shoulder, so the brain interprets the irritation as shoulder pain. Other signs of rupture include feeling faint, rapid heartbeat, and pale, clammy skin.
Multiple Factors Often Overlap
In many cases, no single cause explains a tubal pregnancy. A person might have had a mild chlamydia infection years ago, smoke occasionally, and have subtle tubal abnormalities they’ve never been aware of. These factors compound each other. Scarring from infection narrows the tube, smoking slows its muscular contractions, and a structural irregularity provides a surface for the egg to latch onto. In about half of tubal pregnancies, no clear risk factor is ever identified. The tube may have microscopic damage or functional changes that don’t show up on any test but are enough to delay an egg’s transit at exactly the wrong moment.