Detecting pregnancy in hamsters isn't always easy. It is important to do so, though. Pregnant female hamsters will attack other hamsters (particularly male hamsters), so it is critical to isolate the expectant mother hamster as soon as you recognise that she's pregnant. Failing to do so can lead to injuries or death due to fighting. Hamster behaviour is often one of the best indicators of a pregnancy, but you might also notice weight gain. A Syrian hamster's pregnancy lasts 16 to 18 days; a dwarf hamster's pregnancy lasts 18 to 21 days. Signs likely won't show up for the first 10 days.
Hamsters (even male hamsters) will often gather bedding into their sleep area. It's normal hamster behaviour. However, if a female hamster starts doing this excessively, it could be a sign of pregnancy. If she never had an interest in pulling a nest together and suddenly starts doing it, that would be a good indicator, as well. If your hamster has always been a nester, this might be hard to detect.
Increased Appetite and Thirst
If you notice your female hamster start to eat more than usual, this could mean that she is pregnant. Many hamsters experience an increased appetite during their pregnancy. Drinking more water than usual could also be a sign that your hamster is pregnant. A hamster's nutritional needs increase during pregnancy and her diet is likely to change to compensate. Be alert to your hamster's eating and drinking habits.
Another possible indicator of hamster pregnancy is food hoarding. Just like nesting, though, this is often a matter of degree. That's because hoarding food is natural behaviour for any hamster. It's not uncommon to find a hamster's sleep area loaded full of stored food. If she starts to store more food in her sleeping area than usual, though, this, too, could be a sign of pregnancy.
Pregnant hamsters will often bite. This can apply to hamsters that don't normally bite. This is particularly true if you are doing something near the hamster's nest or she is being held. They become very protective of their nests and their bodies because of the maternal instinct. Part of the reason they will bite when held is due to discomfort caused by the pregnancy.
Aggressive behaviour is common in pregnant hamsters, particularly in protecting their nest area. They will often attack other hamsters that come near their nest. Of course, this is another that can be difficult to spot because some types of hamsters (Syrian) should never be housed with other hamsters as adults because they will fight and potentially kill one another. However, if you have a hamster that normally gets along with other hamsters that suddenly turns mean, she might be pregnant.