The causes of spontaneous abortion at 40 days of poor fetal development may be embryonic reasons, the mother’s own reasons and environmental reasons. 1. Embryonic causes: mostly caused by chromosomal abnormalities in the embryo, commonly caused by chromosomal structure or number abnormalities, such as trisomy 13, trisomy 18, trisomy 21, chromosomal balanced translocation, deletion and so on. 2. The mother’s own reasons: (1) Systemic diseases: when the pregnant woman herself suffers from severe infections, hyperthermia, severe anemia or thrombophilia, it may lead to spontaneous abortion. (2) Endocrine abnormalities: such as the combination of luteal insufficiency, polycystic ovary syndrome, hypothyroidism, hyperprolactinemia, etc., can also lead to spontaneous abortion. (3) abnormal immune function: such as the combination of systemic lupus erythematosus, dry syndrome, antiphospholipid syndrome and other rheumatic immune diseases can also lead to spontaneous abortion. 3. Environmental causes: when too much exposure to formaldehyde, benzene, radiation and other chemical substances, may also lead to miscarriage.