A threatened abortion is the most common type of spontaneous abortion and is characterized by vaginal bleeding in early pregnancy with a closed cervix and a viable fetus. It represents a sign that miscarriage may occur but with potential for continuation of pregnancy. Unlike other types such as missed, inevitable, or septic abortion, threatened abortion does not involve cervical dilation or passage of products of conception. Early recognition is crucial for monitoring and supportive care to improve pregnancy outcomes.

Reference: Williams Obstetrics, Cunningham et al., 25th Edition.