Niddah 3:2-3

Niddah 3:2

If a woman miscarries something red that resembles a rind, a hair, dust or gnats, she should put it in water; if it dissolves, she is ritually unclean and if not, she is clean. Let’s say that a woman miscarries something that resembles fish, locusts, insects or vermin. If it was accompanied by blood, she is ritually unclean but if not, she is clean. Let’s say that a woman miscarries something that resembles a domestic or wild animal, or a bird, whether kosher or non-kosher. If it was male, she observes the procedure for after delivering a boy; if it was female, she observes the procedure for after delivering a girl; if the gender is unknown, she observes both the procedure for after delivering a boy and the procedure for after delivering a girl. This is the opinion of Rabbi Meir. The Sages, however, say that anything that doesn’t resemble a person is not considered a fetus.

Niddah 3:3

If a woman miscarries a sac full of water, blood or multicolored shreds, she need not assume that there was a fetus. If the impression of a fetus can be seen, then she must observe both the procedure for after delivering a boy and the procedure for after delivering a girl.