Pig Gestation Calculator — 3-3-3 Rule & Farrowing Date Predictor

How Pig Gestation Works

Pig gestation = 114 days (3 months, 3 weeks, 3 days)

The pig gestation period is one of the most consistent in the animal kingdom: 114 ± 2 days. The traditional farmer's rule '3 months, 3 weeks, 3 days' (3 × 30 + 3 × 7 + 3 = 114) has been used for centuries and is remarkably accurate across all pig breeds. Modern commercial sows average 114.5 days.

3-3-3

Farmer's Rule — 3 months (90 days) + 3 weeks (21 days) + 3 days = 114 days. Accurate to ±2 days for all breeds. This mnemonic has been passed down through generations of pig farmers and remains the gold standard for predicting farrowing dates.

Parity Effect

Number of Previous Litters — Gilts (first pregnancy) may farrow ~1 day earlier. Mature sows (parity 3–5) are most consistent. Older sows (6+) may carry 1 day longer.

How to Use

1. Enter the first breeding date or AI date.
2. Select the breed type and parity for more precise estimation.
3. The due date is calculated using the 114-day rule.
4. Review the pregnancy milestones to know what to expect at each stage.
5. Prepare the farrowing crate by day 107–110 — move the sow early to reduce stress.

Quick Reference

Common Uses

• •Commercial pig farming — scheduling farrowing to manage all-in/all-out batch production and barn utilization.
• •Small farm planning — preparing farrowing pens, heat lamps, and supplies before piglets arrive.
• •Veterinary scheduling — timing pregnancy checks, vaccinations, and farrowing supervision.
• •4H and FFA projects — students tracking their breeding projects for fairs and livestock competitions.
• •Potbelly pig pet owners — preparing for an unexpected litter and finding homes for piglets.

Understanding the Result

Pig gestation is remarkably consistent across all breeds — 114 days, give or take 2 days. This precision is unusual among livestock and makes the pig farmer's job more predictable than cattle or horse breeding. The '3-3-3 rule' (3 months, 3 weeks, 3 days) is one of the most reliable mnemonics in animal husbandry. Day 1–12: fertilized embryos migrate through the uterus. Day 12–14: implantation — embryos must be spaced evenly in both uterine horns, with at least 4 embryos present total for the pregnancy to continue. Day 21–28: ultrasound can confirm pregnancy. Day 30–70: organogenesis and steady growth. Day 70+: the 'growth phase' — 70% of fetal weight is gained in the last 30 days. Day 107–110: move the sow to the farrowing crate to reduce stress and prevent crushing of newborn piglets. Day 113: colostrum appears — farrowing is 24–48 hours away. Signs of imminent farrowing: restless nesting behavior, milk letdown, and a drop in body temperature (~1°C). Most sows farrow in the evening or at night. The pig industry has optimized gestation to maximize productivity: a commercial sow produces 2.2–2.5 litters per year with 10–14 piglets per litter — over 30 piglets annually.

Worked Examples

Frequently Asked Questions

What is the 3-3-3 rule for pig gestation?

The 3-3-3 rule is the farmer's mnemonic for pig gestation: 3 months (90 days) + 3 weeks (21 days) + 3 days = 114 days. Simply take the breeding date, count forward 3 months, then 3 weeks, then 3 days. Example: bred January 1 → April 1 (3 months) → April 22 (3 weeks) → April 25 (3 days). This method has been used for generations and is accurate to ±2 days. Modern research confirms the average is exactly 114.5 days across all breeds.

How soon after farrowing can a sow be bred again?

Sows return to estrus (standing heat) 3–7 days after weaning piglets. In commercial systems, piglets are weaned at 21–28 days, so the sow returns to estrus around day 25–32 postpartum. Breeding at this first estrus gives a farrowing interval of ~140 days (114 + 28) or about 2.6 litters per year. Some producers use "skip-a-heat" — waiting until the second post-weaning estrus — for better litter size in gilts or thin sows.

What are the signs that a sow is about to farrow?

Key signs: (1) Nesting behavior — the sow gathers bedding material 24–48 hours before farrowing. (2) Udder development — "bagging up" begins 7–10 days before, with colostrum present 24 hours before. (3) Vulvar swelling — 2–3 days before. (4) Milk letdown — squeeze a teat and milk flows; farrowing is within 12–24 hours. (5) Body temperature — drops ~1°C (1.8°F) 12–24 hours before farrowing. (6) Restlessness and lying on her side alternating with standing. Piglets arrive every 10–20 minutes, with the entire litter delivered in 2–6 hours.

How many piglets are in a typical litter?

Commercial crossbred sows average 12–14 total born, with 10–12 born alive. Gilts average 9–11 piglets in their first litter. Litter size increases with parity, peaking at parity 3–5 (13–16 total). After parity 6–7, litter size gradually declines. Heritage breeds average 6–10 piglets. The largest recorded litter: 37 piglets (Australia, 2021). Factors affecting litter size: genetics, nutrition, parity, season (slightly larger in spring/fall), and breeding timing relative to ovulation.

How do I calculate the farrowing date for a potbelly pig?

Potbelly (miniature) pigs have the same 114-day gestation as commercial pigs. The 3-3-3 rule works identically. The key difference is litter size: potbellies average 4–8 piglets vs 10–14 for commercial breeds. Signs of pregnancy are less obvious in potbellies — weight gain and udder development may not be visible until the final 2–3 weeks. Ultrasound confirmation at day 25–30 is recommended for pet pig owners who may not be certain of breeding dates.

Pro Tips

• →Piglets should nurse within 30 minutes of birth to receive colostrum — the first milk rich in antibodies. Piglets that don't nurse quickly have significantly lower survival rates.
• →The farrowing environment should be 32–35°C (90–95°F) for piglets but 18–20°C (65–70°F) for the sow. Use heat lamps or heating pads in a creep area — the sow and piglets have incompatible temperature needs.
• →If farrowing exceeds 6 hours or piglets arrive more than 45 minutes apart, consider veterinary intervention. Dystocia (difficult birth) is less common in pigs than cattle but does occur, especially in gilts and obese sows.
• →Record keeping is your most powerful tool: a sow card tracking each litter (total born, born alive, stillborns, weaning weight) lets you cull underperforming sows and select replacement gilts from the best mothers.

Limitations to Know

• •This calculator uses the standard 114-day gestation period for swine. Individual sows may farrow 1–2 days earlier or later depending on breed, parity, and health status.
• •Factors not accounted for: premature farrowing due to illness, heat stress, or mycotoxin-contaminated feed; breed-specific variations beyond the listed breed types; and the effect of unusually large litters (14+ piglets) which sometimes trigger earlier farrowing.
• •Environmental stressors such as extreme temperatures, overcrowding, and poor nutrition can alter gestation length and are not modeled by this calculator.
• •This is a planning tool, not a substitute for veterinary monitoring. Always consult a swine veterinarian for high-value breeding stock or when complications arise.

Sources

• Wikipedia — Pig Gestation
• Pig Gestation — Engineering Reference