So maybe your rat escaped and went canoodling in the bucks’ cage, or maybe you picked a suspiciously round female out from a feeder tank, or perhaps you’re breeding rats intentionally (if so, I hope you’re a reputable, responsible breeder)– either way, you need to know if she’s pregnant! Since rat gestation is only 21 days, it’s important to know when your doe is pregnant and when she’s due to deliver. The last thing you want is a surprise litter in a cage full of other females, who might steal the babies and prevent them from nursing!
If there is any chance you may have a pregnant rat, you need a gram scale. A decent one can be had for $10-$15 at any grocery store or big box outlet, usually in the baking section. Set a large carton on the scale, zero it out, then set the rat in the carton. Wait until the numbers stop fluctuating, then write down the number the scale settles on.
When trying to determine pregnancy, make sure to weigh your rat at the same time of day every day, as exactly as possible. I usually weigh at 10 PM. Rats’ weight will change hour by hour, so the time of day is very important. I have some rats who have as much as a 10 gram difference in their weight from the morning to the evening each day.
A good rule of thumb is that if a female rat gains half an ounce (a little over 14 grams) in the first three days after mating, she is pregnant. However, some rats may gain only a couple of grams each day until the second week of pregnancy. If your female rat gains weight steadily, with the daily gain increasing over time, she is likely pregnant. In the third and final week of pregnancy, weight gain will be very rapid.
You must, must, must weigh any rat suspected to be pregnant, in grams, daily. However, other signs will be present and can corroborate the conclusions drawn from weight gain, or lack thereof.
Most obviously, a pregnant rat will appear to have a rounded belly and protruding nipples as delivery draws near. You may also note some behavioral changes, such as increased food hoarding or nesting behavior. Reduced activity is normal in the second and third week of pregnancy, but complete lethargy is not. Some pregnant does may become very friendly, while others become shy and avoid handling.
When a pregnant rat is about to deliver her litter, she will build a noticeable nest and may not come out even to eat or drink. Some blood spotting on the bedding may occur when labor begins. If it continues for more than a couple of hours and no babies appear, your rat may need to see an emergency vet. However, the majority of pregnant rats deliver easily without human intervention.
If you have a round female rat with a large nest about 21 days after her last exposure to a male, the best thing to do is leave her alone, turn out the lights, and listen for squeaks! Trust me, if live babies are delivered, you will know. They are not quiet!