Trisomy 18 is a genetic disorder that is mainly caused by an abnormal number of chromosomes. There are many causes of chromosomal abnormalities, such as genetics, maternal age, and chemical and physical factors. Trisomy 18 is a hereditary disorder that appears from the fetal period and can greatly affect the intellectual as well as physical development of the fetus, mainly due to chromosomal abnormalities. Normally, there are 23 pairs of chromosomes in the human body, of which there are two chromosome 18s, but if the patient has an extra chromosome 18, then it means that he or she has trisomy 18. There are many reasons for this phenomenon, such as heredity and mutation. If there is a chromosomal abnormality in the parents, it is likely to be passed on to the next generation. Not only that, as a pregnant woman ages, the quality of her eggs decreases, which increases the chances of chromosomal abnormalities. In addition, physical and chemical factors, such as pre-pregnancy exposure to X-rays, pesticides and chemical drugs, may also cause chromosomal aberrations in the fetus, which may lead to diseases. Therefore, it is important for pregnant women to go to the hospital for regular checkups during pregnancy to detect diseases in time. If the disease has already been diagnosed, then it is recommended to terminate the pregnancy so as not to jeopardize the health of the fetus.