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Reproduction is an essential pillar in pig farming, and careful selection of breeding stock (males and females) is fundamental to ensuring reproductive efficiency and the quality of progeny. Hyperprolific females have increased the average number of piglets per litter, but this advancement is associated with a higher number of piglets with low birth weight (Figure 1), a consequence of intrauterine growth restriction (IUGR) (Tummaruk et al., 2023; Patterson & Foxcroft, 2021).
Figure 1. Two piglets from the same litter, born at term, one with normal weight (below) and one born with IUGR (above). Source: Personal archive.
What is IUGR?
IUGR is characterized by the inability of the fetus to reach its growth potential during a full-term gestation. This condition differs from prematurity and often occurs in multiparous species such as pigs, mainly due to limited uterine capacity and inefficient placental structure.
Criteria for identifying affected piglets
Intrauterine growth restriction (IUGR) can be identified in live animals through phenotypic characteristics, such as a “dolphin-like" head shape (Figure 2), many folds in the snout region, and a head that is proportionally larger than the rest of the body. In dead animals, during necropsy, the brain and liver are removed, and both organs are weighed to obtain the brain weight to liver weight ratio, which is greater than 1 when the animal has suffered IUGR.
High prolificacy intensifies intrauterine competition for space and nutrients, which impairs fetal development and results in weight variability within litters (Town et al., 2004; Almeida & Alvarenga, 2022; Matheson et al., 2018).
Why does this occur?
The porcine placenta, which is epitheliochorial, has six cell layers between the mother and the fetus (Figure 3). The formation of placental folds with different types of trophoblast cells aims to increase the maternal-fetal exchange area. However, morphological and functional changes in this organ, such as low vascularization and structural malformation, are the main causes of IUGR (Bjorkman, 1973; Vallet, 2013; Town et al., 2004; Stenhouse, 2019; König, 2021).
Recent studies indicate that larger fetuses tend to have more periendometrial vessels and larger endometrial glands, which favors their nutrition and development. In addition, there is sexual dimorphism in placental morphology, with males requiring greater nutritional intake. This may result in higher birth weight in males. Such differences suggest that fetal weight is directly related to placental quality and the efficiency of the uterine environment (Franco, 2021).
IUGR has a well-documented impact on zootechnical parameters, such as higher pre-weaning mortality, poorer feed conversion, and lower lean meat deposition. A higher mortality rate (about 18%) was observed in IUGR animals compared to normal animals (about 7%) throughout the stages of the production cycle (birth, weaning, growing-finishing, and slaughter) (Alvarenga et al., 2012). However, its effects on reproductive development are still poorly explored, although relevant (Alvarenga et al., 2013; Felicioni et al., 2020).
Therefore, this article will address the impacts of IUGR on the reproductive characteristics of female and male pigs in adulthood.
In females, inadequate distribution of nutrients in the uterus compromises the development of reproductive organs (Table 1), which have low physiological priority. Such changes compromise cyclicity and fertility, resulting in females with lower reproductive performance and smaller litters (Bronson, 1994; Ross & Desai, 2013; Almeida et al., 2017; Cardoso et al., 2022; Almeida et al., 2017a; Almeida et al., 2017b; Costermans et al., 2020).
In males, birth weight directly influences testicular structure and semen quality (Table 1). This results in lower semen production, lower sperm concentration, and fewer insemination doses per ejaculate. In addition, these males have smaller and less developed testicles throughout their lives, even though part of the cellular structure is restored over time (Knox, 2016; Auler et al., 2016; Sacramento et al., 2022).
Table 1. Reproductive characteristics observed in IUGR piglets.
| Females | Males |
| Morphological changes in the ovaries, such as an increase in the number of primordial follicles | Shorter length of seminiferous tubules |
| Reduction of mature follicles and atretic follicles | Fewer Sertoli cells |
| Reduction in ovarian size and corpus luteum quality | Lower expression of androgen receptors, despite similar testosterone concentrations |
How to mitigate the effects of IUGR?
During gestation, strategies such as supplementation with functional amino acids, such as arginine, and the use of progestogens, such as Altrenogest, have shown potential in improving uteroplacental function and reducing the incidence of piglets with low birth weights.
A recent study revealed that administering Altrenogest at the end of lactation (Pires et al., 2023) resulted in a greater number of endometrial glands per area, due to higher levels of circulating progesterone, which led to fewer stillbirths in the subsequent parity. Other studies have also shown lower rates of IUGR and a better post-weaning hormonal profile (Van Ginneken et al., 2022; Almeida et al., 2000; Schenkel et al., 2010).
Differentiated nutritional plans, such as bump feeding (gradual increase in feed quantity during gestation), have also demonstrated benefits in litter uniformity and lactation performance. Supplementation with functional immunomodulatory amino acids (capable of regulating key metabolic pathways to enhance body growth and productive and reproductive performance in animals, such as lysine, methionine, threonine, tryptophan, valine, and carnitine) throughout gestation resulted in higher weights and numbers of liveborn piglets (Ferreira et al., 2021; Alkmim, 2023).
Post-farrowing interventions include nutritional supplementation during the nursery, growth, and finishing phases, as well as actions aimed at improving the intestinal functionality of IUGR piglets. There are also studies focused on the genetic selection of sows with greater uterine capacity, aiming to permanently reduce the incidence of the condition (Hu et al., 2017; Muns et al., 2017; Viott et al., 2018; Santos et al., 2022; Xiong et al., 2020; Zhang et al., 2020; Matheson et al., 2018).
In summary, intrauterine growth restriction is a complex condition with lasting effects that compromise both the productivity and fertility of pigs. Modern pig farming, which invests heavily in reproductive efficiency, needs to address this problem with robust strategies, ranging from genetic selection and nutritional management of breeding stock to individualized support for affected piglets. Addressing IUGR requires a multidisciplinary approach, integrating genetics, nutrition, reproductive physiology, and management (Felicioni et al., 2020; Santos et al., 2022).