Pig Gestation Calculator (Swine)

Article — Pig Gestation Calculator (Swine)

Swine gestation calculator: pig farrowing date and timeline

Pig gestation averages exactly 114 days — the classic 3-3-3 rule (3 months, 3 weeks, 3 days). Normal farrowing window is 112 to 116 days for 90% of sows. All commercial pig breeds gestate within 2 days of this average, so breed has almost no effect on the expected farrowing date.

Pig gestation is the most consistent in livestock. A cow's gestation varies by 10 days across breeds; a pig's varies by less than 2. The 3-3-3 mnemonic works because it really is the answer. Below: how to plan the timeline around it.

The pig gestation 3-3-3 rule

Three months plus three weeks plus three days equals 114 days. The phrasing is a hand-me-down from European hog farmers and works as well today as in 1900. Some extension materials add a fourth "3" for the 21-day lactation phase or the 3-week breeding cycle, giving 3-3-3-3.

The number itself comes from millions of sow records in USDA's National Animal Health Monitoring System. The standard deviation is ~1.5 days, meaning 95% of sows farrow between day 111 and day 117. Outside that band, suspect a breeding-date error rather than a true outlier.

Pig gestation stages explained

Four phases. Days 0–40 is embryonic — cell division, blastocyst migration through the uterine horns, and implantation around day 14. The embryo elongates to a 1-mm filament on day 16 before attaching. Phase II (days 30–77) is organogenesis: ossification begins around day 35, organs form by day 50.

Phase III (days 70–90) is rapid fetal growth and the development of fetal immunocompetence — piglets can mount their own immune responses from about day 75. Phase IV (days 90–114) is energy accumulation in fat depots and final mammary development. Milk letdown 6–24 hours before farrowing is the most reliable late sign.

Breeding the sow before pig gestation

The sow cycle is 18–24 days, averaging 21. Standing heat (estrus) lasts 24–72 hours in mature sows and 24–48 hours in gilts. Ovulation occurs 24–48 hours after the start of standing heat. Artificial insemination is timed for 12–24 hours after detection — the sperm need to be in the oviduct when ovulation happens.

Most commercial breeders inseminate twice during a heat: once at 12 hours and again at 24 hours. This raises conception rate from ~70% (single AI) to ~90% (double AI) and improves litter size by 0.5–1.0 piglets on average.

Pig gestation feeding

Energy demand is flat for the first 90 days — a maintenance ration of 4–5 lb feed per sow per day. Underfeeding here doesn't help (lean sows have lower conception) and overfeeding wastes money. The shift comes at day 90: increase ration to 6–7 lb daily as the fetuses gain most of their birthweight in the last 25 days.

Some farms use a "flushing" protocol — boost feed by 20% in the 10–14 days before breeding to push ovulation rate. It adds 0.5 piglets per litter on average. Don't continue flushing after breeding; high feed in early gestation lowers embryo survival.

Pregnancy check in swine

Ultrasound scanning between day 28 and day 35 confirms pregnancy with ~95% accuracy. Earlier than 28, the embryos are too small. Later than 35 is fine for confirmation but loses the management value of an early result.

Doppler ultrasound detects fetal heartbeat reliably from day 21. Real-time B-mode ultrasound is the standard tool — handheld units cost $1,500–$3,000 and pay for themselves in a 30-sow herd in one year by reducing non-productive days (a non-pregnant sow eats but doesn't earn).

• Heat detection accuracy: 92–95% with daily boar exposure
• Conception rate (good herd): 88–92% per service
• Average litter size (sows): 12–15 born alive (modern genetics)
• Average litter size (gilts): 10–11 born alive
• Stillborn rate (target): <6% of total born
• Preweaning mortality (target): <12% of born alive

Farrowing preparation

Move pregnant sows to the farrowing crate or pen at day 107, exactly one week before due. Disinfect the crate, set the heat lamp for piglets (90–95°F / 32–35°C target in the creep area for the first 3 days), and stock supplies: iodine for navel dipping, towels, lubricant, syringes for iron injection at day 3, and tooth/tail clippers if used.

Watch for vulvar swelling at day 110, milk letdown at day 113, and nest-building behavior 6–12 hours before farrowing. Total farrowing takes 2–6 hours from first piglet to last. Average interval between piglets is 15 minutes. Intervals over 45 minutes signal a problem.

Pig gestation warning signs

Call the vet if any of these happen: no farrowing by day 118; more than 45 minutes between piglets during active labor; foul-smelling discharge before farrowing (suspect uterine infection); a sow off-feed for more than a day in late gestation (possible pregnancy toxemia in obese sows); or a piglet stuck in the birth canal you can't reach.

Post-farrowing management is as important as gestation management. The sow needs cool water within reach, a quiet environment, and ad-lib feed by day 3 postpartum. Lactation peaks around day 21 at 8–12 liters of milk per day — among the highest milk yields of any mammal scaled to body weight. Sows fall off body condition fast; some lose 25–35 kg of weight by weaning.

Piglet management starts at birth. Dip navels in iodine within minutes of delivery to prevent joint ill. Inject 200 mg iron at day 1–3 — sow milk is iron-deficient and piglets become anemic within a week without supplementation. Pigs born under 1 kg often need cross-fostering to a smaller litter where they can compete for teats. Wean at 21–28 days for commercial herds, 5–6 weeks for outdoor or heritage operations. The 3-3-3-3 rule (gestation, lactation, breeding, rest) maps onto a 21-day cycle that lets a sow produce 2.4 litters per year — about 30 piglets annually.