Mouse wean date calculator
Enter the day a litter was born or the day you set up a breeding pair. You get the dates that follow, and a calendar file so the reminders land in your own calendar.
- Oct 19, 2026 nowBorn (P0)
Count the pups and note any losses over the next few days.
- Nov 7, 2026 – Nov 9, 2026 in 19 daysNext litter, if the male stayed in
Females can mate again the day they give birth (postpartum estrus), so a second litter can arrive before this one is weaned.
- Nov 9, 2026 in 21 daysWean (P21)
Separate the pups from the parents, by sex.
- Nov 16, 2026 in 28 daysLatest usual wean (P28)
For small or slow pups, if your protocol allows a later wean.
Opens in Google Calendar, Outlook or Apple Calendar.
About these dates
When should mice be weaned?
Most labs wean at 21 days old (P21). Small or slow-growing pups, and some strains, are often left with the mother until P28 if the IACUC protocol allows it. Check your own protocol and facility rules.
How long is mouse gestation?
About 19 to 21 days from mating. A vaginal plug the morning after mating marks day 0.5; counting 19 days from a pairing gives the earliest likely birth date.
Why can a second litter arrive before the first is weaned?
Female mice come into heat again within hours of giving birth (postpartum estrus). If the male stays in the cage, the next litter can be born about 19–21 days after the last one, while the first is still nursing. Take the male out before the birth to avoid it.
When should I take the male out?
For non-continuous breeding, remove him about 18 days after setting up the pair, before the litter is born. Leave him in for continuous breeding, which gives more litters but overlapping ones.
These are common lab conventions, not veterinary advice. Your IACUC protocol and facility rules come first.
Timing pregnancies from a plug? The timed mating calculator gives embryonic days (E-days) and when to set up pairs for a stage.
How it’s worked out
All dates are calendar days counted from the date you enter (day 0).
- From a birth (P0): wean = birth + wean age (21 days unless you change it); latest usual wean = birth + 28; next litter if the male stayed in = birth + 19 to 21 days (postpartum estrus).
- From a pairing: plug checks on days 1–4; take the male out on day 18; litter expected from day 19 (19 days of gestation if she mated the first night) to day 25 (allowing for a pair that mated later in the week); wean = litter dates + wean age.
Example: a litter born on October 3 is weaned at P21 on October 24. If the male stayed in, the next litter can arrive October 22–24, before this one is weaned.
Sources
- The Jackson Laboratory: Timed pregnant mice
- The Jackson Laboratory: Colony planning
- NC3Rs: Breeding and colony management FAQs
Last checked against these sources on . Found a mistake? Tell us.