One ramification of this distinction regards animals. Mi’de’oraisa, only milk from a mother animal is forbidden to be cooked with meat. If one has milk from a non-mother animal, it will only be forbidden mi’de’rabbanan to be cooked with meat. If an animal is pregnant is it considered a mother yet, or will we have to wait until it gives birth in order for it to be considered a mother? According to what we have explained, the animal will not be considered a mother until it actually gives birth. Yet even if the animal dies during labor, its offspring will have a mother.
So regarding Dinah, who was conceived in Rachel’s womb, Rachel is considered to be her mother. Thus Shimon, who was conceived in Leah’s womb, was permitted to marry Dinah.