Final answer:
The majority of causes for 1st trimester spontaneous abortions are genetic abnormalities, with chromosomal issues like Trisomy 21 leading to early miscarriages. Environmental factors and maternal health contribute to a smaller percentage of cases. Tubal pregnancies are another, albeit less frequent, cause due to physical blockages in the fallopian tubes.
Step-by-step explanation:
Causes of 1st Trimester Spontaneous Abortions
The majority of causes of 1st trimester spontaneous abortions, commonly known as miscarriages, are attributed to genetic abnormalities. Chromosomal anomalies, such as the most common trisomy in humans, can result in the natural termination of a pregnancy during its early stages. Specifically, conditions like Trisomy 21, though viable in births, often lead to miscarriage within the 1st trimester. In addition to genetic factors, environmental influences such as maternal health, infections, and toxicants account for approximately 10% of complications leading to spontaneous abortions. Moreover, tubal pregnancies, which may occur due to scar tissue caused by sexually transmitted bacterial infections, can impede the progress of the embryo, leading to miscarriage, although half may resolve spontaneously. On the other hand, placenta previa, which happens when the embryo implants near the internal os of the cervix, is a less common cause and usually presents issues later in pregnancy.