How Gestation Shapes Piglet Outcomes

Gestation in the pig averages approximately 114 days, but the conditions experienced during this period vary among fetuses within a litter. As litter size has increased, individual piglets develop under different levels of uterine and placental support, resulting in variation in growth and physiological capacity at birth.

These differences are not random. Piglet size, vigor, and ability to survive after birth are strongly influenced by developmental conditions established during pregnancy. As a result, many of the challenges observed at farrowing originate before birth. Understanding how fetal development occurs provides a foundation for improving both piglet survival and overall productivity.

 Management Implications:

• Not all piglets are equally developed at birth, even within the same litter
• Differences in birthweight and vigor are often established during gestation
• Management at farrowing improves survival but does not eliminate biological variation
• Strategies that support fetal development can reduce the number of at-risk piglets

Early gestation (Day 0–30): embryo survival and placental establishment

Early gestation is a critical period for embryo survival and the establishment of pregnancy.

Following fertilization, embryos undergo rapid cell division as they move from the oviduct into the uterus. During this time, they transition from spherical blastocysts to elongated filamentous structures, a process that occurs around Days 10–12 and is essential for establishing contact with the uterine environment. Around Days 11–12, embryos signal to the sow to support pregnancy. This process occurs over a short window and depends on coordinated development among embryos.

As the number of embryos increases, uterine capacity begins to influence development, and competition for space and access to the uterine environment becomes more pronounced. These early differences contribute to variation in placental development and fetal growth later in gestation [1,2]. Embryonic losses are most common during this period and often reflect failure of embryos to establish adequate interaction with the uterus or underlying developmental abnormalities.

Management during this stage should aim to support successful pregnancy establishment and minimize disruptions around the time of embryo attachment, as this is a critical window for embryo survival.

Figure 1: Illustration of the Rapid Proliferation and Morphological Changes of the Early Pig Embryo. Created using Biorender

Mid-gestation (Day 30–70): establishing placental capacity

Mid-gestation is characterized by continued placental development and the establishment of major fetal organ systems.

Placental growth during this period establishes the capacity for nutrient and oxygen transfer to the fetus. Differences in placental size and development among littermates are evident by mid-gestation and are closely associated with subsequent fetal size [3,4].

Fetal growth during this period is modest compared to later stages but is essential for establishing the structural and functional foundation needed for rapid growth in late gestation, including the development of major organ systems. Because placental development precedes major differences in fetal weight, constraints during this stage often determine which fetuses will experience growth restriction later in pregnancy [2,3].

Around Day 28–35, pregnancy can be confirmed using ultrasound, allowing producers to make management decisions such as grouping sows or adjusting feeding strategies. Fetal loss during this stage may result in mummified fetuses observed at farrowing, reflecting developmental disruption after early placental establishment.

Management and nutritional strategies that support placental development during this period may influence the capacity for fetal growth later in gestation.

Late gestation (Day 70–114): rapid fetal growth and increasing nutritional demands

Late gestation represents the period of rapid fetal growth, with the majority of fetal weight gain occurring during this stage.

As fetuses grow, nutrient and oxygen demand increase substantially, placing greater pressure on the sow and the utero-placental system. In large litters, this results in increased competition among fetuses. Because uterine capacity and placental development are finite, not all fetuses are supported equally, even under well-managed conditions, leading to increased variation in fetal size and the emergence of low birthweight or growth-restricted piglets [5,6].

Placental function during this period is a key determinant of fetal growth. Fetuses supported by smaller or less developed placentas receive fewer nutrients and may exhibit intrauterine growth restriction (IUGR), characterized by reduced body weight and altered tissue development [5,6]. These differences reflect limitations in placental development and function in addition to maternal nutrient supply.

Nutritional management during late gestation is particularly important, as feed intake must support both rapid fetal growth and mammary development. Maintaining appropriate body condition is essential, as both under- and over-conditioned sows may have reduced capacity to support optimal fetal development and lactation performance.

Figure 2: Developmental Milestones of Pig Gestation. Created using Biorender

Variation within the litter: development is not uniform

Although gestation is often described as a series of developmental milestones, fetuses within a litter do not develop under identical conditions.

Differences in embryo development, uterine position, placental growth, and nutrient availability result in variation in fetal size and physiological condition at birth. Within-litter variation is a consistent feature of modern swine production and is strongly associated with piglet survival [6,7].

As a result, some piglets are born with reduced energy reserves, impaired thermoregulatory capacity, and lower vitality, placing them at greater risk during the neonatal period.

How gestation shapes what you see at birth

Differences observed at birth, such as variation in piglet size and vigor, reflect underlying differences in fetal development during gestation, particularly in placental function and nutrient supply. As a result, management during and after farrowing often addresses the consequences of variation established before birth rather than its origin. While practices such as warming, colostrum intake, split suckling, and cross-fostering can improve survival, they do not alter the developmental conditions experienced during gestation.

Newborn piglet. Photo source: Adobe Stock Images

From gestation to piglet survival

The conditions experienced during gestation directly influence outcomes after birth.

Piglets born with reduced body weight or compromised development are less able to:

• maintain body temperature
• establish energy balance
• successfully compete for colostrum

These limitations contribute to the cascade of events that lead to pre-weaning mortality, particularly within the first few days of life [6–8].

Improving piglet survival therefore requires both effective management at farrowing and strategies that support fetal development during gestation.

 Key takeaways

• Fetal development in pigs is influenced by uterine capacity and placental function
• Development is not uniform across littermates
• Most variation in birthweight and viability originates during gestation
• Late gestation is a critical period of rapid growth and increasing nutrient demand
• Piglet survival is strongly influenced by developmental conditions established before birth

Understanding these processes provides a foundation for improving both reproductive efficiency and piglet survival in modern swine systems.

Key management windows during gestation

• Early gestation (Day 0–30): support embryo survival and pregnancy establishment
• Mid-gestation (Day 30–70): maintain conditions that support placental development
• Late gestation (Day 70–114): meet increasing nutrient demands and prepare for farrowing

References

1. Foxcroft, G.R. et al. (2006). Journal of Animal Science, 84(E. Suppl), E105–E112.
2. Freking, B.A. et al. (2007). Journal of Animal Science, 85, 2093–2103.
3. Vallet, J.L. and Freking, B.A. (2007). Journal of Animal Science, 85, 3267–3275.
4. Vonnahme, K.A. and Ford, S.P. (2004). Journal of Physiology, 554, 194–201.
5. Wu, G. et al. (2006). Journal of Animal Science, 84, 2316–2337.
6. Quiniou, N. et al. (2002). Livestock Production Science, 78, 63–70.
7. Milligan, B.N. et al. (2002). Livestock Production Science, 76, 181–191.
8. Devillers, N. et al. (2011). Animal, 5, 1605–1612.