The Mishna at the end of yesterday’s daf 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 Mishna from Massekhet Oholot (7:4) that teaches that when the fetus dies in its mother’s womb, it imparts ritual impurity from when the womb opens, but only when the head of the fetus is round like a skein of yarn.
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 Ḥullin 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 Mishna 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 skein.”
The Gemara offers a number of suggestions to define “the size of a skein,” noting that there are a number of different skeins. Ultimately, Ravin comes from Israel and quotes Rabbi Yoḥanan as ruling that the different size skeins relate to different situations of ritual defilement, with the smallest of them the size relating to human beings and larger ones to animals.
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 earth 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. Rabbeinu Gershom explains that a clod of earth the size of a large skein used by sack makers 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.