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Rats can be sexually mature before six weeks of age, so if your female rat was not separated at around five weeks old from all male rats, including family members, she may be pregnant. Predicting the delivery date will be difficult if you did not see the mating or if your new pet rat turns out to be pregnant. The gestation period for rats varies from 21 to 24 days, and during pregnancy and fetal development, a female's body experiences many changes.
One of the first signs that your rat has mated successfully is the appearance in her vagina of a copulatory mucous plug, formed from mucus and secretions of the male rats’ accessory sex glands. The plug holds the sperm in the female’s vagina and also may prevent impregnation by other males. The presence of a plug is not definitive evidence that a female rat is pregnant, but it indicates that she probably is.
Rats go into heat every four to five days throughout the year, and a typical cycle occurs overnight and lasts about 12 hours. If your rat’s cycles are trackable and she misses one or more, your rat might be pregnant. Skipping only one cycle may indicate that your rat was pregnant, but lost the litter. Some female rats do not have visible cycles, so you will have to rely on other signs to help you determine if she is pregnant.
An indication of pregnancy is an increase in your rat’s appetite, which occurs because the pregnant female’s body requires extra nutrients to maintain her health and strength, and to nourish her growing fetuses. Some pregnant rats steal food from other females with whom they used to share meals. Determined to have enough rations, pregnant rats may be more protective of their food and seek out hiding places in which to stash it. Your rat will rely on you to supply her with healthy snacks, plus fresh water and balanced food staples, such as lab blocks.
Being pregnant alters a female rat’s hormonal balance, and this can result in behavioral changes. Pregnancy may explain why your usually calm and sweet female rat is being territorial and aggressive with her rat and human friends and family. During pregnancy, a hyperactive female may take on a calm demeanor or begin sleeping more, especially as birthing day approaches. Your fun-loving rat may stop playing, or your submissive rat may become dominant. Following pregnancy, birth and the first few weeks of providing for her newborns, your female rat’s pre-pregnancy personality is likely to return.
Response to Male Rats
Once they become pregnant, female rats usually lose interest in males. Even though your female may have exhibited strong reactions to the presence of males in her environment or the odor of a male rat on your skin and clothes, your pregnant rat is unlikely to sniff at, seek out, show curiosity or respond at all to the scent of male rats.
If you are able to weigh your female rat before mating you will have a baseline from which to measure her weight gain. A typical female rat whose gestation is progressing properly will gain weight immediately after conception, then slowly add weight for most of the pregnancy and finally experience a significant gain at the end of her pregnancy. Weight variations will occur throughout pregnancy; however, if time passes with no weight gain or if your rat is losing weight, a serious problem may exist. Your rat should be assessed by a veterinarian who is familiar with rat care.
When your rat is about two weeks pregnant, the size of her abdomen may begin to grow. The third week is when you will easily see her mid-section expanding, and you also may notice movement. Avoid palpating your rat’s abdomen to feel the fetuses, because they are very fragile, and even gentle touching can harm them. During the final stage of pregnancy, your rat is likely to look pear-shaped as her hips widen and her lower abdomen becomes significantly rounder.
Another sign of pregnancy occurs by the third week of pregnancy, when the female rat’s nipples become more pronounced and visible. About a week before she gives birth, your pregnant rat will lose some of the fur that surrounds her nipples. The purpose of the fur loss is to expose the nipples so the new babies can suckle more easily.
A few days before a pregnant rat gives birth, certain hormone levels increase, while others decrease. Hormonal changes intensify a rat’s maternal behavior, which triggers her natural instinct to nest. Nest building by your female rat clearly indicates that her babies will be arriving very soon. Some females build simple nests, while others assemble elaborate nests to hide their newborns. You can help the expectant mother by providing her with a single level cage, some hiding boxes and safe materials she can use to create her nest.
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