From a Buddhist perspective, why is spontaneous abortion?

Hello, I guess there are several possibilities

1. The fetus’s own karma causes it to not have a fetal shape and cause miscarriage

2. Maybe the fetus is originally It is for revenge, that is, the fetus is an enemy and a creditor, but if the pregnant woman or her family does more good deeds, the bad karma will be eliminated, and the fetus may take advantage of the merit of the good deeds to escape.

3. It may also be the pregnant woman's own karma, such as frequent abortions in many lives, leading to miscarriage.

For reference only, I hope you will refrain from doing evil and practice good deeds. I wish you a happy life and believe in the Buddha and recite the Buddha's name. Resolve bad karma and escape from life and death.