It is rather amazing when birth weights of all the kids in a litter are the same. More often, there is some amount of variantion in birth weights and sizes. There may occasionally be very small and weak kids born that appear to be premature, compared to their littermates.

There is a small possibility that very tiny kids were conceived on a different breeding date than the normal-sized siblings (Bred & Pregnant & Bred Again). More often, it is the area of attachment of their placentas within the uterus during gestation that makes them small. This can happen in two areas. A fetus crowded into the rear portion of the uterine horn will have little or no room for more placental attachment when litter mates are in front of them, and 2) There are many fewer areas of placental attachment at and around the cervix. Kids in these positions suffer from limited uterine nutrition from the mother because they have smaller placentas. This resoning will be borne out when very small kids are born first or last.

Two places to check to see if this is a possibility for a runt are: 1) the size of its placenta and the number of cotyledonary attachments (dark-colored lumps on the placenta) and the thickness of its umbilical cord, and 2) whether or not the amount of tooth eruption is normal. Teeth are barely at the gum line in bucklings, but the first two incisors have already erupted in doelings.

A kinked or twisted umbilicus during very late pregnancy or the birth process can also result in a small newborn.