The Mishnah at the end of yesterday’s daf (=page) discusses the status of an animal that miscarries and how that affects the status of the next offspring that is born. This leads the Gemara to a discussion of miscarriages in general, and their status in the realm of Jewish law.
The Gemara quotes a Mishnah from Masechet Oholot (7:4) that teaches that when the fetus is stillborn, the opening of the uterus, that is, the time when there is ritual defilement in the house, is not until the embryo on leaving the uterus forms a round head like a coil.
As long as the stillborn fetus remains in the mother, there is no ritual defilement, since this is a situation of tum’ah belu’ah – “swallowed defilement” – which does not ritually defile (see Hullin daf 71). Once the uterus opens, we view it as if the fetus had been born and there is ritual defilement in the entire house. The Mishnah teaches that this is only true in a case where the fetus had developed to the extent that its head is “the size of a coil.”
The Gemara offers a number of suggestions to define “the size of a coil,” noting that there are a number of different coils. Ultimately, Ravin comes from Israel and quotes Rabbi Yohanan as ruling that the different size coils relate to different situations of ritual defilement, with the smallest of them the size relating to human beings, larger ones to animals and the large-size coil of the sack-carriers to “a clod of imported clay.”
There is a Rabbinic injunction on fields that have been plowed where there once was a grave, that anyone who walks through them or even touches a clod of dirt from them, becomes ritually defiled, as if he touched a bone from a dead body. When the Sages in Babylonia saw that the non-Jews buried their dead in a haphazard manner, they decreed that all lands outside of Israel had a level of ritual defilement on them, so that anyone who walks there or touches a clod of earth from there becomes defiled. Rabbenu Gershom explains that “a clod of imported clay” the size of a coil of the sack-carriers has that status, since there may be a bone in that earth, but if it is smaller than that size, there is no need for concern.