Breeding Small Pets: Should You, and What It Involves

This page is for owners wondering whether to breed rabbits, guinea pigs or small rodents, and for anyone who has ended up with an unplanned pregnancy. The honest starting point is that most pet owners shouldn’t breed. The RWAF points to the “thousands of unwanted rabbits in rescue centres”, and the RSPCA warns that rat pups are “difficult to find good homes for”. If you still want to go ahead, the figures and risks below show what you’re taking on.

Reproduction at a glance

All figures are from Merck. Where Merck’s owner pages, its species chapters and its rodent data table disagree, all of them are shown.

Species	Sexual maturity	Gestation	Litter size	Weaning	Separate sexes by
Rabbit	Small breeds 3.5–4 months; medium 4–4.5; giant 6–9	31–33 days	Medium does often have 12–15 (production figures)	About 6 weeks	10–12 weeks, when the testicles descend (Merck); castrate then (RWAF)
Guinea pig	10–12 weeks (data table)	63–69 days	1–7, usually about 3	3 weeks; data table says 30 days	No age given; a 2024 case report puts puberty at 55–70 days (Brazilian Journal of Veterinary Medicine), so separate at weaning
Rat	Males 6–10 weeks, females 8–12	21–23 days	8–18 (owner page and species chapter); 6–13 average (data table)	About 21 days	At maturity (Merck); “as soon as possible after weaning” (RSPCA)
Mouse	6–8 weeks	19–21 days	10–12 (owner page); 5–12 (chapter and table)	About 21 days	6–8 weeks
Hamster	Breeds best from 10 weeks to 15 months; Syrians can reproduce at 7–8 weeks	16–22 days by type; Syrians 16–17	6–8 (owner page); 2–16, average 9 (chapter)	21–28 days	Before 8 weeks
Gerbil	Males 70–84 days; females about 30 days after the vaginal opening forms at 40–60 days	24–26 days; 27+ if nursing; up to 48 if mated after giving birth	1–8 (owner page); 3–7 (chapter); 1–12 (table)	About day 25	Before about 10 weeks (male maturity)
Chinchilla	8 months	About 111 days	1–6, average 2; usually 2 litters a year	42–56 days	Before 8 months

Rat litter size. Both of Merck’s text pages give 8–18 pups, which matches the figure this page used before. Merck’s own Biological Data of Pet Rodents table gives an average of 6–13, taken from a 2021 clinical textbook. Plan for the upper figure: you would need to find homes for up to 18 pups from one litter. Species detail is on the rat, hamster, gerbil and chinchilla pages.

Three risks that catch owners out

Postpartum oestrus: the second litter you didn’t plan. A doe rabbit can conceive 24 hours after giving birth (Merck). Mice and rats come into heat straight after giving birth. Implantation is delayed while they nurse, but Merck notes that opposite-sex pairs left together can produce a litter every 3–5 weeks. Female mice can have up to 15 litters a year. Gerbils start mating again almost immediately (Merck). The male must leave before the birth, not after it. For rats, Merck recommends at least 2 months’ rest between litters.

Guinea pig pelvic fusion. The sow’s pelvic joint is fibrocartilage that widens at birth (Merck). With age it stiffens, “making it harder for the pelvis to spread”, and Merck describes C-sections in guinea pigs as risky with poor survival for the mother. A 2024 case report states that in females older than 8 months the joint has mineralised and widening is “significantly impaired”. It describes an 8-month-old first-time sow that laboured for 12 hours and lost pups (Brazilian Journal of Veterinary Medicine). In practice, a sow that hasn’t given birth by about 8 months is a high-risk first-time mother, and the safe choice is not to breed her at all.

Hamster and chinchilla pairing aggression. Merck notes that hamsters, especially females, can become aggressive, and advises using older males. Watch closely right after introduction so the female doesn’t injure the male. Syrians otherwise live alone (RSPCA), so any pairing is brief and supervised. Female chinchillas are larger and more aggressive than males. They fend off “unsuitable” males by biting, often taking pieces of ears and toes, and older females commonly kill young males housed with them (Merck). Introductions for non-breeding pairs are covered in introducing new pets.

Selling the young: the law in England

Under the Animal Welfare (Licensing of Activities Involving Animals) (England) Regulations 2018, “selling animals as pets… in the course of a business” is a licensable activity. That includes keeping animals with a view to selling them (Schedule 1, Part 2). Your local authority is the licensing body. In deciding whether you’re a business, it must consider whether you sell “with a view to making a profit” or earn “any commission or fee” (Schedule 1, Part 1). A hobby breeder who regularly advertises litters for profit can therefore need a licence. These Regulations cover England only.

Should you breed at all? A checklist

Breed only if you can answer yes to every line:

• You have a named home for every young animal before mating. For rats, that means up to 18 per litter (Merck).
• Both parents are healthy and free of inherited problems. Merck warns some hamsters carry inherited disease, and chinchillas with dental problems “should not be bred”.
• You have an exotics vet lined up for a caesarean, and you can pay for it.
• You can house the sexes apart from the ages in the table, with separate cages for every weaned litter.
• You’ve checked whether the licensing rules apply to you.

Mistakes that end in welfare problems

• Leaving the male in “to help”. Postpartum oestrus means a second pregnancy while the first litter is still nursing (Merck).
• Breeding a guinea pig sow for the first time after about 8 months. The risk is dystocia and a C-section with poor maternal survival (Merck; Brazilian Journal of Veterinary Medicine).
• Checking on newborns. Merck advises at least 7 days without disturbance. In hamsters, cannibalism accounts for nearly all deaths before weaning, and handling the nest or young is a trigger (Merck).
• Keeping a hamster litter warm “enough”. Cannibalism rises when the room drops below 50°F (10°C) (Merck).
• Leaving young rabbits unsexed. A buck castrated within days of his testicles descending never becomes fertile (RWAF), so book it for 10–12 weeks.