Although rabbits have become a cultural symbol of fertility, mice deserve the stereotype as well. A mouse's gestation period is only 21 days, and she can give birth to litters of up to nine babies, which means the population size can explode in just a matter of months. Because of this exceptional fecundity, mixing male and female mice into the same cage can be dangerous--unless, of course, you want babies. Whether it was an accident or a choice, you can tell within days whether or not your mouse has been impregnated.
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Look for the"'plug." After mice copulate, a white plug forms in the female mouse's vagina, which prevents other males from mating with her. Within hours, this plug falls out and can be found in the cage, although it usually dissolves within a day's time. If you suspect that your mice have mated, look for the plug to confirm this fact. When mice mate, it usually leads to pregnancy, as they only copulate when the female is ovulating.
Watch for nest-building behavior. When a female mouse is pregnant, she'll instinctively begin building a nest for the birth. If your cage has sawdust, dirt or other pliable material, watch to see if she uses it to build a nest for her young.
Track her weight. Because of their small body size, female mice balloon noticeably during their pregnancy. Within 15 days, you should see the first faint bulges develop in her stomach. By day 21, it will be clear that she's pregnant.