Compare the two side by side: Note the bucks' testicles have already started to drop. On all the pictures you can see the navel. It is in the centre of the stomach.
This can be confused with nipples, but bucks do not have nipples and there is always more than one! See if you can see it on all the pictures. Luckily rats rarely have unbiblical hernias; I have only seen one in 15 years. When they do it does not appear to do any harm.
It is at this stage that you can sex the rats by taking each kitten in turn and making two piles, one of bucks and one of does, you can then compare them and make sure that each pile only contains one sex.
Even if you still cannot work it out from these photos, you will at least know how many of each you have! If you look closely at the doe in the pictures you will see the vagina is "closed", it will remain so until they are sexually receptive. Droppings at this age are still small and can appear yellowish in colour due to the doe's milk.
As the doe still removes and eats them at this age, don't worry if while you have them out the nest one appears and does not fall away by itself, the doe will remove it when you put them back. As you can see even at one day old you can see that the bucks and does look very different.
The testicles show as a slight bulge already. You can see the navel and the milk band on both sexes. It can sometimes take up to a day for all the kittens to have a full milk band, it will come and go as the baby digests the food, so you will not always see a full belly on each kitten.
You should, however, be able to see it at least once for each kitten during a 24 hour period. But please do not disturb the mother more than is needed.
Most ratty mums are very good about you playing with their babies, but she will want to get on with feeding her babies without your help! I sex my litters when they are born, with practice you will be able to do so with ease. But do re-check as the babies grow! I do hope this guide has been a help. I will add more pictures on different things to look for and abnormalities, on these pages as soon as I can.