Keritot, Chapter One, Mishnah Seven
1) If a woman had five doubtful genital discharges or five doubtful births, she needs to bring only one offering, and she may eat sacrifices [immediately], and she is not liable to bring the other [offerings].
2) If a woman had five certain births, or five certain genital discharges, she brings one offering and may then eat sacrifices [immediately], and she is liable to bring the other offerings.
3) It once happened in Jerusalem that the price of a pair of doves rose to a golden denar. Rabban Shimon ben Gamaliel said: By this sanctuary, I shall not go to sleep tonight before they cost but a [silver] denar! Then he entered the court and taught: if a woman had five certain births or five certain genital discharges she needs to bring only one offering, and she may then eat sacrifices, and she is not liable to bring the other [offerings]. Thereupon the price of a pair of birds stood at a quarter of a [silver] denar each.
Section one: There are two situations that are described here.
1) A woman had genital discharge for three consecutive days once a month for five months and she doesnt know if these occurred during her menstrual cycle, in which case she was not a zavah and does not need to bring a sacrifice, or not during her menstrual cycle and she is a zavah does need to bring a sacrifice.
2) She had five miscarriages and she doesnt know whether what she miscarried counts as a birth and she must bring a sacrifice or doesnt count as a birth and she does not bring a sacrifice.
In both of these cases, the woman might be liable for as many as five sacrifices (each consisting of an olah and a hatat) or she might not be liable at all. The rule in this case is that she needs to bring only one sacrifice and then she can eat any sacrificial meat, as is always the case when a woman brings a sacrifice for being a zavah or for giving birth. While she can, if she wants, bring four more sacrifices, she need not do so.
Section two: However, if she had five certain cases of genital discharge or five certain births, then she must bring all five sacrifices. After she brings the first sacrifice she can already resume eating sacrifices.
Section three: This section contains a fascinating story. Because of the previous halakhah that demanded that a woman bring a sacrifice for every certain birth or certain genital discharge, the price of the birds used for the sacrifices went all the way up to a golden denar for a pair. A golden denar is the equivalent of 25 silver denars. Market forces were working in favor of those selling the birds and would have discouraged women from bringing the sacrifices that they so frequently had to bring. To combat this, Rabban Shimon ben Gamaliel boldly changed the halakhah so that a woman would have to bring only one sacrifice, even if she had had five certain births or discharges. This new halakhah immediately brought the price down to ¼ of a denar.
The actions of Rabban Shimon ben Gamaliel are one of the clearest examples of a rabbi adapting the halakhah to fit the needs of the Jews of his time. The halakhah to Rabban Shimon ben Gamaliel is not supposed to be something that is simply too costly to fulfill. When he saw that the cost of sacrifices was becoming prohibitive, he didnt refrain and simply say, What can I do? Thats what the law demands. Rather he acted to ensure that what the law did demand was something that people could reasonably observe.