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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.
If 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 sugya begins with the disintegrated fetus expelled as a bloody mass. Rabbi Eliezer ben Yaakov rules that the mass must be buried and that the mother is exempt from any later offspring being treated as a firstborn, yet Rabbi Ḥiyya teaches that the mass transmits neither impurity by touch nor by carrying. The tension is obvious. If a fetus was present strongly enough to count as the mother’s first offspring, why does its dead tissue not convey the impurity of neveilah? Rabbi Yoḥanan answers with the principle of bittul berov: the fetal flesh is a minority within the larger mass of blood and other material and is therefore nullified for purposes of impurity.