当受精卵于子宫腔之外着床，便发生异位妊娠，约150例妊娠中会发生一例异位妊娠，大部分的异位妊娠不会维持很长时间，但是输卵管妊娠可持续至妊娠6周左右，输卵管妊娠破裂导致的急性出血常使患者休克，危及生命。
异位妊娠模式图 | 输卵管妊娠:双胞胎(大体) | 破裂切除标本(大体) | 出血(镜下) | 示血凝块和胎盘绒毛(镜下)
Ectopic pregnancies occur when the fertilized ovum implants outside of the uterine fundus. About 1 in 150 pregnancies results in ectopic implantation. Most cannot be sustained at extrauterine sites. However, a tubal ectopic pregnancy, as diagrammed here, may proceed for several weeks, but the enlargement can rupture the tube and lead to acute, life-threatening bleeding, often about 6 weeks after a previous menstrual period.