From tiny newborn to full-grown adult — the developmental milestones week by week.
Guinea pig pups grow up fast. They’re born remarkably developed — furred, eyes open, and mobile — weighing roughly 70 to 115 grams, and they reach their full adult size by around six months of age, ending up somewhere around 700 to 1,200 grams depending on sex and breed. The key milestones along the way: they nibble solid food from about two days old, are weaned around three to four weeks, reach sexual maturity startlingly early (males from as young as three to four weeks — which is why males must be separated by three weeks), and are usually ready for rehoming around six to eight weeks. Tracking their weight as they grow is the best way to make sure development is on track. Here’s what happens at each stage.
A Note on Tracking Growth
Before the stages, one practical tip: weigh your pups regularly on a kitchen scale — daily in the first week, then weekly. Growth is the clearest sign that a young guinea pig is healthy and developing well, and a steady upward trend is exactly what you want to see.
Keep in mind that every pig is an individual, and the weights below are approximate. Boars (males) tend to grow larger than sows (females), and there’s natural variation between breeds and individuals. So treat the numbers as general guides, and focus on steady, consistent growth rather than hitting an exact figure. (For the day-to-day care at each stage, see our guides on caring for baby guinea pigs, newborn feeding, and the newborn health checklist.)
Stage 1: Newborn (Birth to About 1 Week)
Guinea pig pups are precocial — born unusually ready for the world. Within hours of birth they have a full coat of hair, open eyes, teeth, and enough coordination to walk, squeak, and popcorn around. A newborn typically weighs around 70 to 115 grams.
In this first week, pups nurse from their mother and begin nibbling solid food from around two days old. A reassuring quirk to know: pups often lose a small amount of weight in the first day or two, then start gaining steadily from around day three or four. This is the most fragile stage, so daily weigh-ins and watching for warm, active, nursing pups matter most here.
Stage 2: Nursing Pup (About 1 to 3 Weeks)
Over these weeks, pups grow rapidly. They continue nursing while eating more and more solid food, becoming increasingly active, curious, and independent as they explore their world and develop their personalities.
This is also the ideal window for gentle handling and socialization — regular, calm handling now helps produce friendly, confident adults. Growth should be steady and obvious week to week. Importantly, this stage is when a critical deadline approaches: pups are nearing both weaning and sexual maturity, so plans for separating the sexes need to be ready.
Stage 3: Weaning (About 3 to 4 Weeks)
By around three to four weeks, pups are weaned — fully transitioned from milk to solid food, and eating independently. A pup is generally ready to wean once it’s growing well and eating solids confidently (often around 150 grams or more, though this varies).
This stage carries the single most important milestone for safety: male pups must be separated from their mother and sisters by around three weeks, because they can become fertile startlingly young and could cause unwanted (and risky) pregnancies. So weaning and sex-separation happen around the same time. Females also approach sexual maturity around now.
Stage 4: Juvenile / Adolescent (About 1 to 6 Months)
Now fully weaned and independent, young guinea pigs enter a long stretch of continued growth and development. Over these months they keep growing steadily toward adult size, their personalities become well established, and — if they’re being rehomed — the usual age is around six to eight weeks, once they’re independent and correctly sexed.
Two things to manage during this stage: sexual maturity is fully reached (so the sexes must stay appropriately separated or, for mixed-sex companionship, males neutered), and the diet gradually transitions from the baby diet (with growth-supporting alfalfa) to the standard adult diet (grass-hay based, with alfalfa phased out and calcium moderated). Keep vitamin C constant throughout.
Stage 5: Adulthood (About 6 Months Onward)
By around six months of age, guinea pigs reach their full adult size, with growth slowing and then stopping. Adult weights typically settle somewhere around 900 to 1,200 grams for boars and 700 to 900 grams for sows, though this varies by breed and individual.
From here, your pig is fully grown and mature, ready for the long, stable stretch of adult life (guinea pigs commonly live 5 to 7 years). Continue weighing periodically — in adulthood, weight monitoring shifts from tracking growth to catching health problems early, since a change in an adult’s weight is often the first sign something’s wrong.
Growth Stages at a Glance
- Newborn (birth–1 week): ~70–115 g; born furred and mobile; nurses and starts solids (~day 2); small early weight dip then gains.
- Nursing pup (1–3 weeks): rapid growth; nursing plus solids; prime socialization window.
- Weaning (3–4 weeks): weaned onto solids; separate male pups by ~3 weeks; sexual maturity begins.
- Juvenile/adolescent (1–6 months): steady growth; sexual maturity reached; rehoming ~6–8 weeks; diet transitions to adult.
- Adult (6 months+): full size (~700–1,200 g); growth stops; fully mature.
Key Milestones to Remember
- ~2 days: pups start nibbling solid food.
- Day 3–4: weight starts climbing after the normal initial dip.
- ~3 weeks: separate male pups (they can be fertile this young!).
- ~3–4 weeks: weaned.
- ~4 weeks: sexual maturity reached.
- ~6–8 weeks: typical rehoming age.
- ~6 months: full adult size.
Key Takeaways
- Pups are born developed — furred, eyes open, and mobile — at roughly 70–115 g, and reach full size around 6 months.
- Weigh regularly to track growth; a steady upward trend (after a normal small early dip) is the goal, and every pig varies.
- Solids start ~day 2, weaning happens around 3–4 weeks, and adult diet is phased in as they grow.
- Sexual maturity comes startlingly early — males from ~3–4 weeks — so separate male pups by around three weeks.
- Rehoming typically happens around 6–8 weeks, once pups are weaned, independent, and correctly sexed.
- Adults reach ~900–1,200 g (boars) or 700–900 g (sows) by about six months, then growth stops.
- Keep weighing into adulthood — the purpose shifts from tracking growth to catching health issues early.
This article is intended as general educational information for guinea pig owners and is not a substitute for professional veterinary advice. Growth figures are approximate and vary by individual, sex, and breed; if a pup isn’t growing steadily or you have concerns, please consult a qualified veterinarian experienced with guinea pigs.