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 affect roughly 1% to 2% of all pregnancies in the United States. The underlying cause is almost always something that slows or blocks the egg’s journey through the tube, giving it time to implant in the wrong place.
How a Normal Pregnancy Reaches the Uterus
After fertilization, a tiny embryo needs to travel the length of the fallopian tube and arrive in the uterus within a narrow window of time. Two forces drive this journey: the rhythmic beating of microscopic hair-like structures called cilia that line the inside of the tube, and the smooth muscle contractions of the tube wall itself. These two systems work together like a conveyor belt, gently pushing the embryo toward the uterus over the course of several days.
Anything that disrupts either of those transport systems, or that physically narrows the tube, can trap the embryo. When the embryo stops moving but keeps developing, it implants into the tube wall. That’s the core mechanism behind every tubal pregnancy, regardless of the specific trigger.
Pelvic Infections and Tubal Scarring
Pelvic inflammatory disease (PID) is the single most well-established cause. PID is an infection of the upper reproductive tract, most often caused by sexually transmitted bacteria like chlamydia or gonorrhea. The infection triggers inflammation that can destroy the delicate ciliated cells lining the inside of the fallopian tube. Over time, the inflammation also produces scar tissue, adhesions, and partial or total blockages within the tube.
Even after the infection clears, the damage remains. A tube that has lost its inner lining of cilia can no longer propel an embryo efficiently. A tube narrowed by scar tissue can physically trap one. Many people with PID-related tubal damage never knew they had the infection, since chlamydia in particular often causes no symptoms at all.
Previous Surgery on or Near the Tubes
Any surgery involving the fallopian tubes increases the risk of a future tubal pregnancy. Tubal ligation (surgical sterilization) is a common example. While the overall failure rate is very low, when sterilization does fail and pregnancy occurs, 15% to 20% of those pregnancies are ectopic. The risk is higher when sterilization is performed during the postpartum period, because the tubes are swollen and congested, making complete closure less reliable.
Tubal ligation reversal carries risk for similar reasons. Reconnecting a tube that was previously cut or banded creates a junction of scar tissue where an embryo can get stuck. Surgery for a prior ectopic pregnancy, removal of ovarian cysts, or any procedure that involves the pelvic area can also leave adhesions on or around the tubes that interfere with normal transport.
Endometriosis
Endometriosis causes tissue similar to the uterine lining to grow in places it shouldn’t, including on or inside the fallopian tubes. This misplaced tissue triggers a chronic inflammatory response. The body sends immune cells to the area and releases inflammatory signals that, over time, cause fibrosis and adhesion formation. The result is structural damage to the tube that mirrors what happens with PID: narrowing, stiffening, and loss of normal function.
Smoking
Smoking roughly doubles the risk of tubal pregnancy, and the connection is well documented. Interestingly, research has found that smoking does not appear to reduce the actual number of cilia in the fallopian tubes or alter the genes responsible for producing them. This means the damage likely works through other pathways, possibly by affecting the muscular contractions of the tube wall or altering the chemical environment inside the tube in ways that encourage premature implantation. The risk increases with the number of cigarettes smoked per day.
Hormonal Disruptions
The fallopian tubes rely on a precise hormonal balance to function. Estrogen drives the rhythmic muscular contractions that help move the embryo along, while progesterone inhibits those contractions. In experiments where estrogen was given to restore tubal contractions, a subsequent dose of progesterone shut them down completely. Any condition or medication that throws off this balance can slow tubal transport enough for an embryo to implant before it reaches the uterus.
This is one reason why certain contraceptive failures carry elevated ectopic risk. Progesterone-based methods work partly by thickening cervical mucus and altering tubal motility. If pregnancy occurs despite these methods, the hormonal environment may have already slowed the embryo’s transit through the tube.
IUDs and Contraceptive Failure
IUDs are highly effective at preventing pregnancy, and current IUD users actually have a lower overall risk of ectopic pregnancy than people using no contraception at all. The confusion arises because of what happens in the rare case of IUD failure. If you do become pregnant with an IUD in place, the odds that the pregnancy is ectopic are dramatically higher, roughly 21 times higher than in someone not using contraception. This is because IUDs are better at preventing implantation inside the uterus than at preventing fertilization in the tube.
Previous IUD use (meaning past, not current) also carries a slight increase in ectopic risk, and the risk rises with the duration of past use. This may reflect low-grade inflammation in the tubes from long-term device use.
IVF and Assisted Reproduction
Ectopic pregnancy occurs in 2% to 5% of pregnancies conceived through assisted reproductive technology, a higher rate than the general population. This seems paradoxical because IVF places the embryo directly into the uterus, bypassing the fallopian tubes entirely. Several factors explain the increased risk.
First, many people undergoing IVF already have tubal disease, which is often why they needed IVF in the first place. Second, the hormonal stimulation used during IVF cycles can impair normal tubal function and alter the receptivity of the uterine lining. Third, the physical act of embryo transfer can trigger abnormal uterine contractions that push the embryo backward into the tube. The volume and pressure of the transfer fluid may also force an embryo through the opening where the tube meets the uterus. People with underlying tubal problems face a compounded risk from both the existing damage and the effects of the IVF process itself.
A Previous Ectopic Pregnancy
Having one tubal pregnancy raises your risk of having another to somewhere between 10% and 27%. This is partly because the original cause, whether it was tubal scarring, endometriosis, or structural abnormalities, is likely still present. Treatment for the first ectopic can also leave additional scar tissue. The recurrence rate is high enough that early monitoring with ultrasound and blood tests is standard in any subsequent pregnancy after an ectopic.
How a Tubal Pregnancy Is Identified
In a healthy early pregnancy, levels of the hormone hCG roughly double every two days. When hCG rises more slowly than expected (less than a 50% increase over 48 hours), plateaus, or follows an irregular pattern, it raises suspicion for an ectopic pregnancy. However, no single hCG reading can rule one out, and some ectopic pregnancies produce normally rising levels.
The key diagnostic tool is transvaginal ultrasound. Once hCG reaches a level between 1,000 and 2,000 IU/L, an intrauterine pregnancy should be visible on ultrasound. If hCG has risen above that threshold and no pregnancy is seen inside the uterus, an ectopic pregnancy is presumed. The combination of serial hCG measurements and ultrasound imaging provides the most reliable diagnosis.