The gestation period in swine, or the pregnancy of a sow, is a biological process that dictates much of the swine farming calendar. Gestation is the time during which the developing embryos and fetuses are carried within the uterus. Understanding this timeline is important for producers, as it impacts herd health, facility scheduling, and nutritional management for a successful birth, known as farrowing.
The Standard Duration and Variation
The average gestation period for a sow is 114 days, with the normal range falling between 113 and 117 days. This duration is often memorized using the mnemonic “3 months, 3 weeks, and 3 days.” This average length is the primary benchmark for predicting farrowing dates and coordinating labor.
The actual length of a sow’s pregnancy can vary slightly based on biological factors. Sows carrying large litters may farrow a day or two earlier due to increased physiological stress. Conversely, a sow with a very small litter might experience a slightly longer gestation period.
The age of the sow, known as parity, can also influence the length of the pregnancy. First-time mothers, or gilts, sometimes have a marginally shorter gestation period compared to older, multiparous sows. Genetic selection for hyper-prolific lines, which produce larger litter sizes, has also shown a trend toward a slight lengthening of the average gestation period in some herds.
Recognizing Pregnancy in Sows
The first and most reliable behavioral sign of successful breeding is the sow’s failure to return to estrus (heat) approximately 18 to 25 days after mating. The estrous cycle typically lasts 21 days; successful breeding results in elevated progesterone levels that maintain the pregnancy. Daily exposure to a boar during this period helps confirm the absence of the standing reflex, which is a clear sign of heat.
Physical signs of pregnancy become more apparent as gestation progresses, though they are less useful for early detection. Abdominal enlargement is noticeable, especially after 80 to 90 days, as the fetuses undergo their rapid final growth phase. The mammary glands will also begin to develop and increase in size, a change that is typically more pronounced in sows that have farrowed before.
For more definitive confirmation, producers often use ultrasound scanning between 28 and 35 days post-breeding. This technology allows for the visualization of fluid-filled embryonic sacs within the uterus, providing an accurate method for pregnancy diagnosis. The vulva of a pregnant sow remains pale and relaxed, in contrast to the swollen and reddened appearance seen during estrus.
Preparing for Farrowing
Management practices shift around day 100 to 110 of gestation to prepare the sow for delivery. It is standard practice to move the sow into the designated farrowing environment, such as a specialized crate or pen, at least four to five days before the expected due date. This allows the sow time to acclimate to the new surroundings, reducing stress that could interfere with labor.
Environmental preparation involves thoroughly cleaning and disinfecting the farrowing quarters to minimize the risk of disease transmission to the newborn piglets. For sows in outdoor systems, this includes moving huts to a clean, dry location and providing fresh nesting material.
Nutritional adjustments are also made in the final days, often involving a reduction in the amount of feed to prevent constipation and congestion of the udder. Producers must still ensure the sow has access to plenty of fresh water.
The Farrowing Process
The onset of labor, or the first stage of farrowing, is signaled by distinct behavioral changes in the sow. She will become restless, frequently changing position, and may exhibit nesting behavior by pawing at the bedding material. A more definitive sign that farrowing is imminent, typically within 24 hours, is the ability to express milk or colostrum from the teats.
The active delivery phase, or second stage of labor, begins with visible abdominal contractions and the expulsion of the first piglet. Piglets are normally born at intervals of 15 to 20 minutes, though this can vary widely, with some arriving in quick succession and others having a longer delay. The entire process typically lasts between two and four hours, but it can extend longer for older sows or those with large litters.
The third stage of farrowing is completed with the expulsion of the afterbirth (placenta), which usually occurs in one mass within one to four hours after the last piglet is born. Observing the sow and intervening if the interval between piglets exceeds an hour is often necessary to prevent stillbirths.