How Long Are Mice Pregnant? Mouse Gestation Period, Signs and What It Means for Your Home

TL;DR

• Mice are pregnant for about 3 weeks. A house mouse usually gives birth about 19 to 21 days after mating.
• A mouse may get pregnant again within hours of giving birth, so a female may have a litter about every 3 to 4 weeks. A mother still nursing may give birth a few days later than one with no pups in the nest.
• Deer mice may carry a little longer, often about 22 to 26 days.
• The clearest signs of a pregnant mouse are often a rounder belly late in the pregnancy and a female busy building a nest. In a house, you're more likely to notice the nest.
• A litter often has about 5 or 6 pups. They are usually weaned at about 3 weeks and can breed from about 5 to 6 weeks after birth.
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

Mice are pregnant for about 3 weeks, but how long mice are pregnant matters less to a homeowner than how quickly one pregnancy follows another. A female can mate again within hours of giving birth, so she is often carrying her next litter while the last one is still in the nest.

That pace turns a quiet corner of the basement into a nursery faster than most people expect. A mouse you spot in the fall may already be pregnant, and her daughters can be old enough to breed before winter is over.

The exact count still shifts by a few days, depending on the species, the source you read and whether the mother is nursing.

How Long Are Mice Pregnant?

Mice are pregnant for about 3 weeks: a house mouse usually gives birth 19 to 21 days after mating. The house mouse is the mouse most likely to be living in your walls, kitchen or garage, so this is the number that matters for most Canadian homes. A few things can stretch the pregnancy slightly, and a female can start a new one almost as soon as she gives birth.

House Mouse Gestation Period

The house mouse gestation period is about 19 to 21 days, according to the Animal Diversity Web (University of Michigan) and the Internet Center for Wildlife Damage Management. UC IPM (University of California) gives 18 to 21 days, and the Government of Alberta gives a slightly longer range. The small differences come from how each source counts (from mating or from conception) and from the conditions the mice live in. For practical purposes, think of it as about 3 weeks.

Gestation in Nursing Mice

Gestation in nursing mice lasts longer. The Animal Diversity Web notes that the pregnancy may be extended by several days if the female is lactating, and the Internet Center for Wildlife Damage Management says females that conceive while nursing may have a slightly longer gestation period. In other words, a mouse that is feeding one litter while pregnant with the next will usually give birth a few days later than a mouse with no pups in the nest.

How Soon a Mouse Can Get Pregnant Again

A mouse can get pregnant again within hours of giving birth. Female house mice may come back into heat within hours of delivering a litter (Animal Diversity Web), which is why the Government of Alberta says that, when conditions are favourable, a female may produce a litter every 3 to 4 weeks. For the full picture of litters per year and how quickly numbers add up, read our guide on how often mice reproduce.

What Is the Mouse Gestation Period for Other Species in Canada?

The mouse gestation period in Canada runs from about 3 weeks for house mice to almost 4 weeks for some native mice. The Government of Alberta names the house mouse and the deer mouse among the most common mice people encounter, and other small rodents tend to follow a similar rhythm. If you are not sure which one you have, our mice identification chart can help.

Deer Mouse

The deer mouse is pregnant for about 22 to 26 days, and it can be several days longer when she is nursing a previous litter (Animal Diversity Web). Deer mice can breed year round, with most breeding in the warmer months, and often have about four to six young per litter. Their range reportedly reaches north to the tree line in Canada. This species matters for your health: Health Canada calls the deer mouse the most common carrier of the deadly hantavirus, so treat any deer mouse nest with care.

White-Footed Mouse

The white-footed mouse has a gestation period of typically about 22 to 28 days (Animal Diversity Web). Pregnancies can run longer in females still nursing their young because implantation of the embryos is delayed. In Canada, its range reportedly reaches from Nova Scotia west to Saskatchewan.

Meadow Vole

The meadow vole is a vole rather than a true mouse, but this small rodent is found in Canada and breeds on a similar schedule. Its pregnancy lasts about 21 days on average, according to the Animal Diversity Web. Litters often hold about 4 to 6 pups.

How Can You Tell if Mice Are Pregnant?

Mice are hard to spot as pregnant in the wild, but the clearest signs are a rounder belly late in the pregnancy and a female busy building a nest. Most of what is known about these signs comes from pet and laboratory mice, so in a house you are more likely to notice the nest than the mouse herself.

Swollen Belly

A swollen belly is the most obvious sign of a pregnant mouse. Pet-care guides reviewed by veterinarians say the swelling often starts around 10 days after mating, but it can appear as late as about 14 days, and it is usually most noticeable in the last week or so. A mouse carrying a small litter may not look pregnant at all.

Visible Nipples

Visible nipples are another sign. As the belly grows, a pregnant mouse's nipples can become more prominent than usual. You would only see this up close, for example on a mouse caught in a live trap.

Nest Building

Nest building is the sign you are most likely to find in your home. UC IPM notes that reproductive females are highly motivated to build nests, which house mice make in sheltered spots from finely shredded paper or other fibrous material. Nests are often about 4 to 6 inches (about 10 to 15 cm) across, and mice tend to prefer spots next to heat sources such as appliance compressors, pilot lights and hot water heaters (Internet Center for Wildlife Damage Management). Shredded paper, insulation or fabric piling up in a hidden corner is a strong clue. Our guide on how to find a mouse nest shows where to look.

Behaviour Changes

Behaviour changes can also point to a pregnancy. A pregnant mouse may avoid other mice and eat more than usual. Smell plays a role too: according to research summarised on Wikipedia, the odour of an unfamiliar male mouse may end a pregnancy, an effect known as the Bruce effect.

What Happens After a Mouse Gives Birth?

A mouse gives birth to a litter of about 5 or 6 pups that are weaned and starting to leave the nest about 3 weeks later. Here is what those first weeks look like for house mice.

Litter Size

A house mouse litter usually has 5 or 6 pups, according to UC IPM and the Government of Alberta. The Animal Diversity Web gives a range of about 3 to 12, litters of up to 14 have been reported in some cases, and UC IPM notes that litter size increases in older females. To estimate how many mice may share one nest, see how many mice live in a nest.

Newborn Pups

Newborn pups are typically born naked and blind. They stay in the mother's nest, where she keeps them warm and feeds them, and they cannot survive on their own at this stage.

The First Two Weeks

The first two weeks are a time of very fast growth. House mouse pups are fully furred after 10 days and open their eyes at 14 days (Animal Diversity Web). By the end of the second week, they look like small versions of adult mice.

Weaning

Weaning happens at about 3 weeks. The Internet Center for Wildlife Damage Management says young mice begin short trips out of the nest and start eating solid food at about 3 weeks, and the Animal Diversity Web notes that young mice stay in the mother's nest until about 21 days old. Shortly after, most leave her territory, although young females are more likely to stay nearby.

Sexual Maturity

Sexual maturity comes 5 to 6 weeks after birth, although it can be delayed until as late as 12 weeks (UC IPM). The Government of Alberta says young mice may reach reproductive maturity in as little as 5 to 7 weeks. To follow a mouse from birth to old age, read our guide to the mouse life cycle.

What Does a 3-Week Mouse Pregnancy Mean for Your Home?

A 3-week mouse pregnancy means a new litter can arrive in your walls less than a month after mice move in, so the time to act is at the first sign. Health Canada puts it simply: mice are prolific breeders, so tackle occasional invaders right away to avoid a severe infestation.

How Fast a Mouse Problem Grows

A mouse problem grows in overlapping waves. Using UC IPM figures, a pregnancy lasts about 3 weeks and the young mature in 5 to 6 weeks, so by our calculation a pregnant female's daughters can be giving birth to their own litters roughly 11 to 12 weeks after she became pregnant. Meanwhile, the mother may have a new litter about every 3 to 4 weeks. Indoors, the Government of Alberta notes that house mice may breed year-round, even during a Canadian winter.

Trap Timing

Trap timing should follow the mouse pregnancy and growth cycle. Health Canada recommends setting 1 trap per metre along walls, or 5 to 10 traps per visible mouse hole, and resetting the traps in 2 to 3 weeks to catch maturing mice. That second round catches the young that were still in the nest during the first round. Our step-by-step guide shows how to set a mouse trap correctly.

Nesting-Material Bait

Nesting material can work better than food as trap bait when females are pregnant or nursing. UC IPM says a small ball of cotton wool or dental floss on the trigger plate can be an effective bait, because reproductive females are so driven to build nests. Health Canada also suggests strong-smelling, sticky foods such as peanut butter.

Bait Stations

Bait stations with rodenticide need extra care. Health Canada advises using poisoned baits only in tamper-proof bait stations wherever children or pets could reach them, and checking that the label shows a Pest Control Products (PCP) number, which means the product is registered for use in Canada. Always follow the label directions exactly.

Cleaning Up a Nest

Cleaning up a nest of pups or a nest left behind needs the same precautions as cleaning up droppings. The Government of Alberta advises opening doors and windows and staying out of the area for at least 30 minutes and wearing rubber gloves. Soak the nest, droppings and any dead mice with one part bleach to nine parts water (or a household disinfectant) for 10 minutes before you pick them up, as the Public Health Agency of Canada and UC IPM advise. Bag and seal everything. Never sweep or vacuum a nest or droppings, because the dust can carry hantavirus. Our guide on how to clean mouse droppings walks through each step.

Sealing Entry Points

Sealing entry points keeps the next pregnant female from moving in. The Government of Alberta recommends sealing openings larger than about 6 mm with gnaw-resistant materials. Do it once trapping has removed the mice inside, so you do not trap a nest of pups in a wall. Our guide on how to find mice entry points shows where to check.

When to Call a Pest Control Professional

Calling a pest control professional makes sense when you find a nest you cannot reach, when you keep catching young mice weeks after you started trapping, or when signs show up in several rooms. A licensed professional can locate nests, set a trapping plan and seal entry points. If you want help, you can get a quote from a recommended local exterminator through Pest Agent's referral form. You can also browse more guides in the mice hub.