I first heard about hysterical pregnancies while watching an episode of Glee. Hysterical pregnancies happen in both humans and other mammals. A hysterical pregnancy in humans, also known as pseudocyesis, is when a person experiences symptoms of pregnancy while they aren't pregnant. Hysterical pregnancies seem to be mainly psychological.
The causes behind false pregnancies aren't known but it is generally thought that they are cause by changes in the endocrine system. The endocrine system secretes hormones, and the changes in the endocrine system causes secretion of hormones which lead to changes in the body similar to those during pregnancy.
The changes in the endocrine system may be because the person desires to be pregnant but can not get pregnant. The disappointment of not being able to get pregnant may lead to effects on the body, causing a false pregnancy. Hysterical pregnancies also frequently happen in people who have been through a traumatic miscarriage.
False pregnancies may also be caused by tumors in the uterus. The tumors affect the menstrual cycle, causing menstruation to be absent. This change in the menstrual cycle may also lead to malfunction in the endocrine system, causing the effects explained above.
The symptoms experienced by the woman during a hysterical pregnancy are extremely similar to those experienced by a woman who is actually pregnant. Some of these symptoms are enlargement of the abdomen, nausea, feelings that something is moving in the stomach, and irregular menstrual cycles. This makes it difficult to diagnose whether a person is experiencing a false pregnancy or not. One way to be sure is to get an ultrasound done. If the person is not really pregnant, there will be no baby.