Ectopic pregnancies most often occur in the fallopian tubes, but they can also implant in the ovary, cervix, abdominal cavity, and even a previous cesarean section scar. About 1 to 2% of all pregnancies are ectopic, meaning the fertilized egg attaches somewhere outside the main cavity of the uterus. Each location carries different risks and can present with different symptoms.
The Fallopian Tube: Where Most Ectopic Pregnancies Happen
Roughly 92 to 98% of all ectopic pregnancies implant somewhere along the fallopian tube. But the tube itself has distinct segments, and where in the tube the embryo attaches matters for both symptoms and treatment.
The ampulla, the wider, curved section closest to the ovary, accounts for about 73% of tubal ectopic pregnancies. This is where fertilization normally happens, so it makes sense that an embryo that fails to travel further would implant here. The isthmus, the narrow middle segment, accounts for roughly 13% of cases. Fimbrial pregnancies, which implant at the finger-like ends of the tube near the ovary, make up about 12%.
The remaining 3% implant in the interstitial segment, where the tube passes through the muscular wall of the uterus. Interstitial pregnancies are particularly dangerous because the surrounding muscle tissue and blood supply allow the pregnancy to grow longer before causing symptoms. Most interstitial pregnancies rupture around 12 weeks of gestation, compared to the 6 to 8 week window typical of other tubal locations. The mortality rate for a ruptured interstitial pregnancy is roughly 2 to 5%, approximately seven times higher than for other ectopic pregnancies. About 40% of all deaths from ectopic pregnancies result from a cornual (interstitial) rupture.
Ovarian Ectopic Pregnancy
In about 3 to 5% of ectopic cases, the embryo implants directly on or within the ovary. This type is difficult to diagnose because on ultrasound an ovarian ectopic pregnancy looks very similar to a hemorrhagic ovarian cyst, which is far more common. Confirming an ovarian ectopic often requires examining tissue after surgical removal rather than relying on imaging alone.
Cervical Ectopic Pregnancy
Cervical ectopic pregnancies, where the embryo implants in the cervical canal below the uterus, are rare, accounting for less than 1% of cases. They were historically caught late, often in the second trimester or when a miscarriage was already underway. Transvaginal ultrasound has improved early detection. On imaging, a cervical pregnancy gives the uterus an hourglass shape, with the gestational sac sitting in the cervical canal rather than the uterine cavity.
Abdominal Ectopic Pregnancy
Abdominal pregnancies make up roughly 1 to 2% of ectopic cases and come in two forms. A primary abdominal pregnancy occurs when a fertilized egg implants directly in the abdominal cavity while the fallopian tubes and ovaries remain intact. A secondary abdominal pregnancy, which is more common, happens when a tubal ectopic ruptures or detaches and the embryo reimplants somewhere else in the abdomen.
The range of possible implantation sites within the abdomen is surprisingly broad. Pregnancies have been reported on the bowel, the omentum (the fatty tissue that drapes over the intestines), the broad ligament supporting the uterus, and the pelvic sidewall. In extremely rare cases, implantation has occurred on the liver, spleen, appendix, and even the lung. Many abdominal pregnancies are only discovered during surgery because they don’t produce the typical ultrasound findings associated with other ectopic locations.
Cesarean Scar Ectopic Pregnancy
This type occurs when an embryo implants in the scar tissue left from a previous cesarean delivery. As cesarean rates have risen worldwide, this once-rare type has become more recognized. The embryo burrows into the defect in the uterine muscle where the surgical incision healed.
There are two patterns of growth. In one, the pregnancy expands inward toward the uterine cavity. These pregnancies can occasionally continue to develop but carry a high risk of the placenta growing abnormally deep into the uterine wall, which can cause severe bleeding later. In the other pattern, the pregnancy grows outward toward the bladder, raising the risk of the scar rupturing and causing internal bleeding. Most cesarean scar pregnancies cause no symptoms early on, though some women experience light vaginal bleeding or mild abdominal pain. Early ultrasound detection is critical because delayed treatment can lead to life-threatening complications.
Heterotopic Pregnancy: Ectopic and Uterine at the Same Time
A heterotopic pregnancy occurs when one embryo implants normally inside the uterus while another implants in an ectopic location, most often the fallopian tube. In natural conception this is exceedingly rare. With IVF and other assisted reproductive technologies, heterotopic pregnancy rates rise to roughly 0.3 to 0.6% of transfer cycles. The increase happens because multiple embryos may be transferred, and despite being placed directly in the uterine cavity, an embryo can migrate into a tube.
Heterotopic pregnancies are easy to miss because a normal intrauterine pregnancy is visible on ultrasound, which can lead clinicians to stop looking for a second implantation site. Persistent or worsening pelvic pain after a confirmed uterine pregnancy, especially after IVF, warrants further evaluation.
Why Ectopic Implantation Happens
Normally, after fertilization in the fallopian tube, tiny hair-like structures lining the tube sweep the embryo toward the uterus over several days. Anything that damages or disrupts these structures, or narrows the tube, can trap the embryo and cause it to implant before reaching the uterus. Pelvic inflammatory disease, prior tubal surgery, endometriosis, and previous ectopic pregnancies all increase risk by causing inflammation or scarring inside the tubes. Smoking also damages the tube’s lining and impairs its ability to move the embryo along.
For ectopic pregnancies outside the tubes, the mechanisms differ. An ovarian pregnancy may result from fertilization happening so close to the ovary that the embryo implants before it ever enters the tube. Abdominal pregnancies can begin when a fertilized egg is released from the end of the tube into the pelvic cavity instead of traveling inward. And cesarean scar pregnancies occur simply because a previous surgical incision created a defect in the uterine wall that a developing embryo can burrow into.
How Location Affects Detection
Transvaginal ultrasound is the primary tool for identifying where an ectopic pregnancy has implanted. The classic finding is an empty uterine cavity paired with a mass or gestational sac outside the uterus. For tubal pregnancies, a ring-shaped structure near but separate from the ovary is a telltale sign. Pressing gently during the scan can help distinguish a tubal mass from the ovary itself.
Interstitial pregnancies show up as a sac sitting eccentrically in the uterine wall, surrounded by a thin layer of muscle less than 5 mm thick. Cervical pregnancies produce the characteristic hourglass appearance. Cesarean scar pregnancies appear as a sac embedded low in the front wall of the uterus at the level of a previous incision, with a thin or absent layer of muscle between the pregnancy and the bladder.
Some locations are far harder to diagnose than others. Ovarian and abdominal pregnancies in particular may evade ultrasound detection entirely and only become apparent when symptoms worsen or during emergency surgery. Free fluid with debris in the pelvis is a warning sign that any ectopic pregnancy may have ruptured and is causing internal bleeding.