An ectopic pregnancy occurs when a fertilized egg implants itself outside of the uterus. Ectopic pregnancies usually occur in a fallopian tube (called a tubal pregnancy). Occasionally, the egg may lodge itself in the ovary, and more rarely, in the cervical canal or the abdominal or pelvic cavities. The fertilized egg doesn't usually grow into a recognizable embryo and can't be transplanted into the uterus.
In a normal pregnancy, the egg is fertilized in the fallopian tube. Little hairs in the fallopian tube move the egg down to the uterus, where it implants itself. If there's scar tissue in the fallopian tube, or it's blocked for some other reason, the fetus will begin to grow outside of the uterus. A woman who has an ectopic pregnancy must have the pregnancy removed because the fetus can't develop properly outside of the uterus and it is dangerous to the woman's health.
Although they're becoming more common in recent years, ectopic pregnancies are generally rare. About 2% of pregnant women will have an ectopic pregnancy.
If you've had an infection in the pelvic region (e.g., pelvic inflammatory disease) or pelvic surgery, or were born with a medical condition that narrowed your fallopian tubes, you have a greater chance of having an ectopic pregnancy. A previous ectopic pregnancy can also increase your risk of a second ectopic pregnancy. An unsuccessful tubal ligation, a sterilization procedure in which the fallopian tubes are cut or blocked, can contribute to the risk of an ectopic pregnancy.
Rarely, ectopic pregnancies have also been linked to the use progesterone-only birth control pills, and the morning-after pill. Women who use intrauterine devices (IUD, a type of birth control), especially those containing progesterone, have a higher risk of having an ectopic pregnancy if they do become pregnant despite using the IUD. Women who were exposed to a medication called diethylstilbestrol* (a synthetic estrogen) in their mother's womb are also at greater risk of ectopic pregnancy.
The use of certain assisted reproductive techniques may also increase the risk of ectopic pregnancy, as can having multiple sexual partners and cigarette smoking.