On rare occasions a woman’s egg is fertilised in the abdominal cavity, or the fertilised egg comes out of the Fallopian tube and the pregnancy progresses in the abdominal cavity, with the placenta and attached embryo implanting onto structures within the abdomen. This is the most extreme form of an ectopic pregnancy.
The pregnancy may continue for many weeks but in due course the placenta is unable to supply the growing foetus with adequate nutrition as it is not implanted into the normal site in the uterus but attaches to whatever structures and organs it comes into contact with in the abdomen.
The woman may be aware that she is pregnant, and her belly swells in a similar way to pregnancy, but the swelling is higher and more irregular than the smooth feeling of a pregnancy in the uterus. When the placenta starts to fail, usually at about 20 weeks of pregnancy, it separates from the structures in the abdomen to which it has been attached, bleeding into the abdomen occurs, and the woman experiences severe pain. At this stage the diagnosis is usually made, and as a result it is very rare for a foetus to survive an abdominal pregnancy.
An operation is necessary to remove the usually dead foetus from the mother’s belly, but a lot of the placenta is often left behind to shrink naturally as attempts to remove it from the structures in which it is embedded can cause serious bleeding.