Hello and thank you for researching this important question!
I'm sorry that no one online last night felt able to discuss this with you.
Hamsters can be difficult to examine, especially when young! Inexperienced pet store personnel can often make mistakes in assigning gender to immature pets.
In female rodents, the openings for urine and dropping are very close together...you may not be able to even see two openings! A female (since she does not need room for testicles) is very pointed at the bottom, with the tail as the tip of the point. A female that is not pregnant will have a trim and slim outline.
In males, the openings for urine and droppings are comparatively farther apart and more easily seen. The "bottom" is flat or rounded with the tail sticking off like the stem of an apple. Older males have larger testicles that can fall down and appear lumpy on the abdomen. This gives them an overall more heafty look and feel...they are somewhat more muscular and larger than females.
Hamsters' testicles do change size from season to season, getting larger in spring and decreasing in size in the fall. This coincides with the breeding season.
Here is a link to a site that has a good photo of a male hammie, and give a lot of care info if your other pet turns out to be pregnant:
Most pet stores do not pay for babies, but are usually willing to take back 4 week-old pups that you cannot find homes for.
Blood coming directly out of the rectum can be a sign of serious illness...have him examined by a licensed veterinarian who is familiar with rodent medicine if you think that the blood came from his bowels. Hamsters can get "bloodied" from fights or prolonged sexual behaviors.
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