A pregnancy is called ectopic when it implants and starts to grow outside the uterus. It can happen on the ovaries, bowels or other areas of the pelvis and abdomen, but the most common site by far are the Fallopian tubes.
The sperm usually meets the egg inside the tube to form an embryo. That embryo then migrates to the uterus where it implants to form the placenta and start to grow the fetus. Sometimes the embryo moves slower than usual because of problems with the tube and is ready to implant before it reaches the uterine cavity. Implantation happens inside the tube and the fetus starts to grow there. Because it is a very small organ it can eventually break causing internal bleeding.
Today the diagnose of pregnancy is done at a very early stage and most ectopic pregnancies are dealt with early on and cases of massive haemorrhage are rare.
Once diagnosed, an ectopic pregnancy needs to be treated and there are two options.
Medical treatment is done with Methotrexate, a chemotherapic drug that stops the growing of embryonic tissue.
Surgical treatment is done through a laparoscopy where depending on size and condition either the tube is incised and the pregnancy removed or the whole tube is removed.