Several factors significantly increase the risk of ectopic pregnancy, where a fertilized egg implants outside the uterus, most often in a fallopian tube. The most important ones involve anything that damages or alters the fallopian tubes: prior pelvic infections, previous ectopic pregnancies, smoking, certain surgeries, and fertility treatments. Some of these risks are preventable, while others help explain why an ectopic pregnancy happened and what to watch for next time.
Prior Pelvic Infections and STIs
Sexually transmitted infections, particularly chlamydia, are one of the strongest and most common risk factors. A large population-based study found that women who tested positive for chlamydia had roughly 90% higher risk of ectopic pregnancy compared to women who tested negative. Chlamydia and gonorrhea can cause pelvic inflammatory disease (PID), an infection of the reproductive organs that creates scar tissue inside the fallopian tubes. That scarring can partially block the tube or damage the tiny hair-like cells that move a fertilized egg toward the uterus.
The tricky part is that chlamydia often causes no symptoms at all. Many women don’t know they’ve had an infection until they’re dealing with fertility problems or an ectopic pregnancy. This is one reason routine chlamydia screening is recommended for sexually active women under 25 and for older women with new or multiple partners.
Previous Ectopic Pregnancy
Having one ectopic pregnancy raises your risk of another to about 10%. After two or more, the risk climbs above 25%. The underlying tubal damage that caused the first ectopic is usually still present, and any surgical treatment may add its own scarring. If you’ve had a previous ectopic pregnancy and become pregnant again, early monitoring with ultrasound can confirm the pregnancy is in the right location before symptoms develop.
Smoking
Cigarette smoking is one of the more overlooked risk factors. Chemicals in cigarette smoke, including compounds called pyridines and phenols, interfere with the fallopian tubes in two key ways: they impair the tube’s ability to pick up the egg after ovulation, and they disrupt the muscular contractions that move the embryo toward the uterus. If the embryo travels too slowly, it can implant in the tube before reaching the uterine cavity.
The risk is dose-dependent. Heavy smokers (those with more than five pack-years of exposure) face roughly four times the odds of tubal problems compared to nonsmokers. Even moderate smoking doubles the risk. Quitting before trying to conceive meaningfully reduces this risk, though existing tubal damage may not fully reverse.
Tubal Surgery and Sterilization Reversal
Any surgery on or near the fallopian tubes can create scar tissue that disrupts normal egg transport. The highest-risk procedures include tubal ligation reversal, surgery to repair a blocked tube, and surgical treatment of a previous ectopic pregnancy. After a sterilization reversal, ectopic pregnancies tend to occur at the site where the tube was reconnected, particularly when the surgeon joins two segments of different diameter. Even surgeries not directly involving the tubes, like removal of an ovarian cyst or treatment for appendicitis, can cause nearby adhesions that affect tubal function.
Fertility Treatments
Pregnancies conceived through IVF and other assisted reproductive technologies carry a notably higher ectopic risk than natural conceptions. The overall rate of ectopic pregnancy in natural conceptions is 1 to 2%, but following assisted reproduction it ranges from 2.1 to 8.6%. In women with a history of tubal problems, the rate can reach 11%. Even with a single embryo transfer, the relative risk of ectopic pregnancy is about six times higher than with natural conception.
This seems counterintuitive since embryos are placed directly into the uterus during IVF. But the embryo can migrate into a fallopian tube after transfer, especially if the tubes are damaged or if the transfer technique places the embryo near a tubal opening. Women undergoing IVF are typically monitored with early ultrasound to confirm proper implantation location.
Endometriosis
Endometriosis, where tissue similar to the uterine lining grows outside the uterus, roughly doubles the risk of ectopic pregnancy. A meta-analysis found an odds ratio between 2.16 and 2.66, meaning women with endometriosis are about two to two-and-a-half times more likely to experience an ectopic implantation. The likely mechanism involves inflammation and scarring around the fallopian tubes and ovaries, which can distort the tubes’ structure and impair their ability to transport an egg normally.
IUDs and Hormonal Contraception
Contraceptives don’t cause ectopic pregnancy in the usual sense. They dramatically reduce your overall chance of getting pregnant at all. But if a pregnancy does occur while you’re using certain methods, the proportion of those pregnancies that turn out to be ectopic is higher than normal.
A recent study published in NEJM Evidence examined ectopic rates across different IUD types. Lower-dose hormonal IUDs had higher ectopic rates than copper IUDs, while the highest-dose hormonal IUD (52 mg levonorgestrel) actually had a lower rate. The absolute numbers are small because IUDs are highly effective at preventing pregnancy in the first place. But if you miss a period or have pregnancy symptoms with an IUD in place, ectopic pregnancy needs to be ruled out promptly.
Progestogen-only pills follow a similar pattern. They’re effective contraceptives overall, but when they fail, the resulting pregnancy is more likely to be ectopic. This is because progestogen slows the movement of eggs through the fallopian tubes. If the pill doesn’t fully prevent ovulation and fertilization occurs, the slowed transport increases the chance of tubal implantation.
Age
Women aged 35 and older face a four- to eight-fold increased risk of ectopic pregnancy compared to younger women. Several factors converge with age: cumulative exposure to STIs, age-related changes in tubal function, and hormonal shifts that can affect how the tubes contract and move eggs along. Age alone isn’t something you can change, but it’s an important reason why providers pay closer attention to early pregnancy location in women over 35.
How Ectopic Pregnancy Is Detected
Most ectopic pregnancies are diagnosed between 6 and 10 weeks of gestation. The key tools are blood tests measuring the pregnancy hormone hCG and transvaginal ultrasound. In a normal early pregnancy, hCG levels roughly double every two days. When levels rise more slowly, plateau, or fail to increase by at least 50% over 48 hours, it raises suspicion for a failing or ectopic pregnancy.
One important point: there’s no single hCG number that rules out an ectopic pregnancy or predicts whether a tube will rupture. In one review of 716 patients with confirmed ectopic pregnancies, 29% of those with very low hCG levels (under 100) still had tubal rupture at the time of surgery. Rupture has been documented across hCG values ranging from 10 to nearly 190,000. This is why serial blood draws and ultrasound imaging together are more reliable than any single test.
When hCG rises above 1,000 to 2,000 and a transvaginal ultrasound still shows no pregnancy in the uterus, that combination is considered strong evidence of an ectopic pregnancy. About 90% of women with ectopic pregnancies and hCG levels above 2,000 ultimately need surgical treatment due to worsening symptoms or rupture.