An ectopic pregnancy happens when a fertilized egg implants outside the uterus, almost always because something prevented it from traveling through the fallopian tube on time. About 1% of all pregnancies are ectopic, with roughly 11 out of every 1,000 pregnancies in the UK falling into this category. The vast majority lodge in the fallopian tube itself, though rare cases occur on the ovary, cervix, in the abdominal cavity, or at the site of a previous cesarean scar. Understanding the specific reasons behind ectopic pregnancies helps explain why some people face higher risk than others.
How the Fallopian Tube Moves an Egg
To understand why things go wrong, it helps to know how the system is supposed to work. The inside of each fallopian tube is lined with tiny hair-like structures called cilia that wave in coordinated patterns, actively sweeping the egg toward the uterus. This ciliary action is thought to play the leading role in transporting both eggs and embryos. The tube’s smooth muscle also contributes two types of contractions: sustained squeezes at key junctions that temporarily hold the egg in place, and rapid, brief pulses that mix the egg with nourishing tubal fluid.
When ciliary movement slows down, when cilia are destroyed, or when muscular contractions lose their normal rhythm, the fertilized egg can stall. If it stays in the tube long enough, it begins implanting right where it is. The tube wall is too thin to support a growing pregnancy, which is what makes ectopic pregnancies dangerous.
Infection and Pelvic Inflammatory Disease
The single most common reason for tubal damage is pelvic inflammatory disease (PID), usually caused by sexually transmitted infections like chlamydia or gonorrhea. These infections don’t just pass through the tubes. They actively destroy the tissue that makes transport possible.
Gonorrhea bacteria invade the non-ciliated cells of the tubal lining but primarily destroy the ciliated cells, causing them to slough off entirely. Exposure to gonorrhea or its toxins leads to a progressive reduction and eventual complete halt of ciliary activity. Chlamydia works similarly, stripping cilia from the tube lining while triggering chronic inflammation that further slows whatever ciliary movement remains. The resulting scar tissue from PID physically narrows or blocks the tube, preventing a fertilized egg from reaching the uterus. Many people with tubal damage from PID never knew they had an infection, since chlamydia in particular often causes no symptoms.
How Smoking Damages the Tubes
Cigarette smoke contains roughly 4,000 compounds known to be toxic, including heavy metals like cadmium and lead, nicotine, and various cancer-causing chemicals. Several of these directly interfere with fallopian tube function through different mechanisms at the same time.
Nicotine alters the muscular contractions of the tube. Smoke compounds also rapidly reduce the beat frequency of cilia, though this effect is at least partially reversible once exposure stops. More concerning is a separate, harder-to-reverse effect: smoke disrupts the sticky connection between cilia tips and the egg, impairing the tube’s ability to grab the egg from the ovary in the first place. Because these two effects are independent of each other, smoking creates a double problem. Even if ciliary beating recovers, the tube’s grip on the egg may not.
Endometriosis and Inflammation
Endometriosis, a condition where tissue similar to the uterine lining grows in places it shouldn’t, increases ectopic pregnancy risk through multiple pathways. The fluid that accumulates in the pelvic cavity of people with even mild-to-moderate endometriosis has a marked inhibitory effect on ciliary beat frequency. Some research suggests this fluid contains large molecules that coat the cilia at the tube’s opening, essentially forming a membrane that blocks the tube’s ability to capture the egg.
Endometriosis also promotes fibrosis, a buildup of scar-like tissue driven by chronic inflammation and hormonal signaling. This fibrosis can physically distort or narrow the tubes. On a molecular level, endometriosis alters the cocktail of signaling molecules, including those involved in cell adhesion, that normally guide an embryo to implant in the uterus. When these signals are disrupted in the tube, the embryo may attach there instead.
Previous Tubal Surgery or Ectopic Pregnancy
Any surgery on or near the fallopian tubes, including procedures to remove a prior ectopic pregnancy, can leave scar tissue that disrupts normal transport. The recurrence rate after one ectopic pregnancy ranges from 10 to 27%. In studies of people who experienced repeat ectopic pregnancies, 69% had one previous episode, while 31% had a history of two or more. Each occurrence further damages the tubes and raises the odds of it happening again.
Ectopic Pregnancy After IVF
Ectopic pregnancy occurs in 2 to 5% of IVF cycles, roughly two to five times the rate in naturally conceived pregnancies. This is surprising, since the embryo is placed directly into the uterus during IVF, bypassing the tubes entirely. Several mechanisms explain why it still happens.
The hormonal stimulation used to produce multiple eggs alters signaling molecules that normally coordinate the interaction between the embryo, tube, and uterine lining. The volume of fluid used during embryo transfer can change pressure inside the uterus, potentially pushing the embryo backward through the tubal opening. Placing the embryo too close to the top of the uterus can trigger strong contractions that propel it into a tube. There’s also a molecular factor: tubal tissue in IVF-related ectopic pregnancies shows higher levels of a specific adhesion molecule (E-cadherin) compared to ectopic pregnancies from natural conception, suggesting the tube may be “stickier” to embryos under hormonal stimulation.
IUDs and the Ectopic Ratio
IUDs are highly effective at preventing pregnancy overall, and they do not increase your absolute risk of ectopic pregnancy compared to using no contraception at all. However, because IUDs are far better at preventing implantation inside the uterus than outside it, the rare pregnancy that does occur with an IUD in place has a disproportionately high chance of being ectopic. In a large European surveillance study, 21 confirmed ectopic pregnancies were reported among IUD users, with copper IUDs accounting for twice as many as hormonal IUDs. If pregnancy is suspected with an IUD in place, the possibility of ectopic implantation should be investigated promptly.
How an Ectopic Pregnancy Is Identified
Early ectopic pregnancies don’t always cause obvious symptoms, so diagnosis often depends on blood tests and ultrasound. A pregnancy hormone (hCG) level that rises too slowly is one of the key warning signs. In a healthy pregnancy, hCG should increase by at least 35% over two days. A slower rise raises concern for either miscarriage or ectopic pregnancy.
About 21% of ectopic pregnancies produce hCG patterns that mimic a normal pregnancy, and 8% show a decline that looks like a miscarriage, making diagnosis tricky in some cases. Ultrasound becomes increasingly useful as hCG levels climb. When hCG is above 3,000 and no pregnancy is visible inside the uterus, roughly one-third of those cases turn out to be ectopic. The combination of hormone trends and imaging findings, sometimes tracked over several days, is what allows doctors to distinguish an ectopic pregnancy from a very early but normal one.
Where Ectopic Pregnancies Implant
The vast majority of ectopic pregnancies, over 90%, implant somewhere along the fallopian tube. The specific location within the tube matters for treatment options and risk level. The ampulla (the wider middle section) is the most common site, followed by the isthmus (the narrow segment closest to the uterus) and the interstitial portion (where the tube passes through the uterine wall). Non-tubal locations include the ovary, cervix, abdominal cavity, and cesarean section scars. These are rarer but can be harder to detect and sometimes more dangerous because of the blood supply in those areas.