Here's a bit of interesting sheep biology that I'm about to explain incredibly badly.....
The ewe's uterus has nobbles on it that look a bit like slices of peperami on a pizza. These are the bits that provide nutrients to the developing lamb. The ewe-end of the lamb's umbilical cord spreads out across an area of the uterus, and covers several of the peperami nodule thingies, to tap into their nutrient supply.
Sometimes, the placement of multiple lambs within the uterus results in certain lambs having access to less than their fair share of nodules. That means that the lamb gets less nutrition within the womb and therefore ends up smaller than its siblings.
If you have a really good look at an afterbirth (turn it inside out so you can see), you'll be able to make out where the umbilical cord spread out as the lamb developed, and also the peperami nodules.
I found all this fascinating when I realised. Your mileage may vary!
