The age profile of the herd is important in relation to fertility as discussed in chapter 5 and the same comments are equally true of the performance of the sow in the farrowing house. Fig.8-9 shows typical results expected in gilts, and older parities in terms of litter size, mortality and pigs reared per sow per year. Stillbirth rates should be noted with increasing age of the sow and piglet mortality which will often exceed 12% from parity 8 onwards. Also with increasing age, birth weights and litter sizes become more variable and there are greater losses associated with poor viable pigs. However older sows may have to be retained to maintain the mating programme and they can be useful in rearing surplus piglets. It should be noted that there are four distinct groups of breeding females in the herd, gilts, second parity, parities 3 to 8 and old sows, all with different performance levels. Each can become a specific problem associated with management and disease.