Early spontaneous abortion (the most prevalent) is usually due to fetal malformations or chromosomal abnormalities. Spontaneous abortion during the last two thirds of pregnancy is more likely to be due to maternal factors, for example abnormalities of the cervix or uterus, insufficient progesterone, sexually transmitted diseases that affect the genital tract, endocrine dysfunction (as in hypothyroidism and diabetes mellitus), or severe emotional trauma. Immunological reactions, in which maternal antibodies mistake the fetus for foreign tissue, have been implicated in recurrent, or habitual spontaneous abortions. It is estimated that at least 20% of all pregnancies end in miscarriage (estimates range from 15% to 75%). Most occur in the first two weeks after conception, and in many cases the mother is not aware of the pregnancy.