The conditions under which sows were maintained in the wild allowed them to gain large amounts of weight in gestation in preparation for the upcoming lactation. During lactation sows nursed their litters for 6 or 7 weeks and the accumulated weight was used for production of milk. Sows used to produce about one litter per year with about 4 to 8 pigs. Sows today have been selected more intensely for certain traits which have made them different in regards to mature body size, reduced appetite potential, higher milk yield, higher lean to fat ratio, higher litter size and growth rates, and more vulnerable to nutritional and environmental factors. Humans have brought about shorter lactation length, earlier breeding, and increased use of AI. These all greatly reduce the margin of error in terms of feeding and management of the modern sow. Feeding strategies in gestation for the sow must not be considered in isolation from those in lactation. An essential part of any strategy to optimize sow reproductive performance is to control weight and back fat gain during gestation and weight and back fat loss in lactation. Large changes in weight and backfat in gestation and lactation over several parities will inevitably lead to an increased culling rate. Excessive weight gain results in heavier sows, which are likely to be too big for stalls and crates. Adaptation of a gestation feeding program based on an estimate of sow weight and a measurement of back fat will reduce the percentage of fat sows and reduce variation in sow weight and back fat levels at farrowing, which should help to reduce sow weight loss in lactation and improve sow reproductive efficiency.