1. Number of pigs born per litter and position in the dominance order (the sow has only so much uterine space and the number of embryos that attach to the placenta wall, then the space between each fetus affects size of the resulting piglet).
2. Nutrition of the gestating sow, especially the last trimester (last 30 days) of pregnancy. (Individual piglet birthweight will increase by around 0.2 kg in respnse to an extra 1 kg of feed given daily throughout the 114-115 days of pregnancy, which is a feed:gain ratio of about 50:1.)
It has been suggested that restricted feeding of sows at any time during pregnancy may, either through compromising the placenta itself or by reducing the flow of nutrients to the fetal load, result in piglets at birth with less than their potential number of muscle fibers (lean tissue resulting in weight gain). These suggestions would point to disadvantage in feeding sows at a low level through any state of pregnancy, and an advantage in more generous pregnancy feed provision. It is far from clear whether this effect is real or supposed, or what the differences might be between feed levels defined as ‘restricted’, ‘adequate’ and ‘excessive’.
3. Parity of the pregnant female. (The birth weight of piglets is greater in multiparous than primiparous sows. As litter size increases, the average birth weight decreases.)
4. Sow genotype (maternal body size, it can well be appreciated that the birth weight of piglets from large breeds of pig is greater than that from small breeds.)
5. It could be said that carry-over effects from lactation feeding (or pre-conception feeding in the case of first litter gilts) would also have some positive effects on birth weight, BUT that would be almost impossible to quantify.