Nest-building behaviour is an important part in the process of pre and post-partum maternal behaviour. Nest building will occur during the last 24 hours before the onset of farrowing and becomes most intense during 12 to 6 hours before farrowing.  Nest building is divided into two phases: one of which is the initial phase of rooting and pawing the ground while the second phase is the collecting, carrying and arranging of the nest material.  One type of animal that does nest building are sows.  The sow will separate from the group and seek a suitable nest site with some shelter from rain and wind, and which has well-drained soil. This nest building behaviour is performed to provide the offspring with shelter, comfort, and thermoregulation. The piglets need an increased environmental temperature otherwise they could die due to the cold.  The nest will provide protection against weather and predators, while keeping the piglets close to the sow and away from the rest of the herd. This ensures they do not get trampled on and that other piglets are not stealing the milk from the sow (food resource).  The nest building behaviour can be influenced by internal and external stimuli. Internal hormonal changes and the completion of one nesting phase are indicators of this maternal behaviour.  The onset of nest building is triggered by the rise in prolactin levels, which is caused by a decrease in progesterone and an increase in prostaglandin. While the gathering of the nest material seems to be regulated more by external stimuli such as temperature or udder comfort.  Nest building ends when the sow gathers softer material and distributes throughout nest by nodding head movements and pawing with the front legs.  The longer time spent on nest building will increase pre-partum oxytocin, allowing for higher piglet weight gain. Nest building is related to the increased litter size, a higher responsiveness to piglet distress, and lower mortality rate in piglets. Letting the sow perform the nest-building behaviour can lead to better health and welfare for both the sow and piglet.