In order to create a comprehensive teshuvah on abortion, one must review the traditional sources of the Torah and Mishnah, form individual opinions, and respond to modern responsa.
Exodus 21:22-23 explains the status of a fetus. It states that if people are fighting and one accidentally strikes a women, who thereby has a miscarriage, but there was no severe damage to the woman, then the man will surely be fined accordingly and the money shall be paid to the woman's husband. However, if a man strikes the women and she dies as a result, then the man shall pay a soul for a soul. In this passage we see that the torah believes capital punishment is mandatory in the case where one person has killed another, "(he) shall pay a soul for a soul." Although the man does not receive capital punishment for killing a fetus, the man is fined for it. Therefore, the torah communicates that although the fetus is not a full human life, the death of one should not be taken lightly.
The above is the Jewish translation of the text. However the Greek translation of this text in the Septuagint that is immensely different from the Jewish one. The change sprouts from a difference in the translation of the Hebrew word "ason." The Jewish translation interprets "ason" to mean, "damage to the mother." Therefore if there is any damage to the mother, besides the miscarriage, then the man pays a life for a life. The Greek translation interprets "ason" to mean, "form." Therefore if the miscarried fetus had already formed, the man will pay "a soul for a soul" because a formed fetus is considered a full human life.
Oholot 7:6 suggests the importance of the mother in relation to the fetus. It states that in the case of a women for whom labor would be life threatening, the doctors must cut the fetus up in the mother's stomach and take it out limb by limb, because the mother's life is more important than the fetus" life.