**1. Introduction**

In the pig, just as in other mammalian species, the process of parturition includes three phase: opening of the cervix (I), expulsion of the fetuses (II) and expulsion of the placentae (III). In the 1990's, the average duration of farrowing was 1.5.-2 hours [1]. Since 1990, there has been a linear increase in both 1) litter size from about 10 piglets in 1990 to close to 20 piglets in 2019 and 2) duration of farrowing from 1.5–2 hours to 7–8 hours (a conclusion based on 20 studies on duration of farrowing, **Figure 1**, [2]). While the described tendency is subject to differences between breeds and management (i.e. farrowing crate vs. free farrowing), the overall tendency is rather convincing. The extended duration of farrowing appears an outcome of intensive breeding for prolificacy in the pig [2].

The increasing litter size presents with an immunological challenge for the sow and especially the piglets [2, 3]. The last 20–30% of the fetuses to be born likely miss out on access to good quality colostrum that declines by 50% already by the 6th hour after the birth of the first piglet [4]. On the other hand, they also have less time to suckle colostrum due to decreased window of opportunity for colostrum intake,

#### **Figure 1.**

*Relationship between litter size and the duration of farrowing in 20 studies from 1992 to 2018 (adapted from Oliviero et al. 2019, reproduction in domestic animals, Wiley-Blackwell).*

increased competition for teats and reduced birth weight. This all may show up later in emergence of diseases in the growing phase of piglets/fattening pigs.

The metabolic challenge related to hyper-prolific sow production model begins in the growing phase of gilts and goes beyond farrowing and lactation. The sow is supposed to eat enough to meet the requirement of growing litters prior to farrowing, which may cause some of the problems seen at around farrowing [5, 6]. In the early part of lactation, sows with large litters loose more energy while producing milk than what they can consume in their feed, ending up in a negative energy balance (NEB) [7, 8].

The growing litter size and intensity of production as such appear as items for welfare concern for the public. This seems to happen regardless of whether those concerns would be warranted or not. However, this review will tackle those items relating to welfare of the hyper-prolific sow model that we know, based on scientific literature, as having reasons to be addressed.
