Final answer:
The main cause of spontaneous/incomplete abortions during the first trimester is genetic abnormalities, particularly chromosomal issues such as trisomy. Trisomy 21 is the most common trisomy in viable births but can also lead to miscarriage in the first trimester. Other causes include maternal health issues, infections, and exposure to certain teratogens.
Step-by-step explanation:
The number one cause of spontaneous/incomplete abortions during the first trimester is genetic abnormalities, particularly chromosomal anomalies. One of the most common types of these anomalies is a condition known as trisomy, where an extra chromosome is present in the embryo. The most frequently occurring trisomy that leads to miscarriage is trisomy 16, though it is rarely diagnosed because the miscarriage typically happens very early in the pregnancy. Another common trisomy associated with spontaneous miscarriage is Trisomy 21, although it is more often recognized in viable births. Other factors that can lead to spontaneous abortion include maternal health issues, infections, and exposure to teratogens like smoking. In the context of tubal or ectopic pregnancies, scar tissue from sexually transmitted infections can lead to a spontaneous termination of the pregnancy if the embryo becomes implanted in the fallopian tube rather than the uterus.