A false pregnancy, hysterical pregnancy or pseudopregnancy is when a mammal shows some of the symptoms associated with a pregnancy, but without being pregnant. False pregnancies commonly occur in dogs and mice. False pregnancy in humans is less common, and usually has psychological causes. False pregnancy is probably caused by changes in the endocrine system of the body, which leads to the secretion of hormones. These hormones are the reason physical changes happen, similar to those during pregnancy. The underlying cause is often mental.