וְאִם־לֹ֨א תִמְצָ֣א יָדָהּ֮ דֵּ֣י שֶׂה֒ וְלָקְחָ֣ה שְׁתֵּֽי־תֹרִ֗ים א֤וֹ שְׁנֵי֙ בְּנֵ֣י יוֹנָ֔ה אֶחָ֥ד לְעֹלָ֖ה וְאֶחָ֣ד לְחַטָּ֑את וְכִפֶּ֥ר עָלֶ֛יהָ הַכֹּהֵ֖ן וְטָהֵֽרָה׃ (פ)
At kung hindi niya kayang magdala ng isang tupa, kukuha siya ng dalawang batu-bato o dalawang inakay na kalapati, ang isa ay bilang handog na susunugin at ang isa ay bilang handog para sa kasalanan; at gagawa ang saserdote ng pagbabayad-sala para sa kanya, at siya ay magiging malinis.
Mishneh Torah, Sanctification of the New Month
There are things for which we don't make a leap year at all; but we do make them a support for a year that needs to become a leap [year] because of the season or because of the Spring [state of the barley] and the fruits of the trees. And these are them: Because of the goats and sheep that are yet to be born or which are few. And because of the chicks that have not [yet grown enough to have] flown. We do not make a leap year because of these – in order that the goats and sheep should be accessible for the Pesach sacrifices; and [that] the chicks be accessible for the offering of being seen (reiah) or for those who are obligated in a sacrifice of fowl. But we do make them a support for the year.
Sefer HaChinukh
It is from the roots of the commandment [that it is] in order that she be aroused by action to give thanks to God, blessed be He, Who saved her from the pangs of her birth, which is a miraculous thing. And also they, may their memory be blessed, said (Niddah 31b) that a woman rushes to swear at the time of her throes that she will no longer [have sexual relations with] a man. And therefore she requires atonement, and hence her sacrifice is called a sin-offering, and it is stated about her, “and the priest shall atone for her” (Leviticus 12:8).