The most important thing to remember when you’ve got a litter is that mother knows best. You have almost no chance of successfully raising mouse babies under about 2 weeks old, and even from that age it’s extremely difficult. If you only remember one thing, remember this: Don’t stress, she knows what she’s doing!
You’ll notice when mum is getting close to giving birth that she’ll work furiously on her nest, and usually spend a lot of time in it. If you’re lucky enough to be present when she chooses to give birth, don’t touch her or the babies, poke around the nest, or disturb the cage at all – just leave her alone to do what she does best. Shortly after birth, you’ll start to hear the little squeaks of babies, which are called pinkies at this stage. They will squeak like this almost until they are weaned primarily for communication with their mother.
Sometimes a mother mouse will kill and eat (or partially eat…) some of her babies. She may do this if the baby is malformed or not healthy, if she’s feeling stressed and nervous, if she’s unwell, or too old or young, or if she has a large litter and she feels she can’t raise them all. Usually any culls will happen at or close to birth, but it can also happen up until about 2 weeks. Occasionally you will get a mouse that just isn’t a very good mother and will kill or abandon her babies, but this is quite rare in my experience. In this situation, the best thing you can do is provide her with a stress-free environment. I would also avoid re-breeding any mouse who excessively culled.
If mum knows and trusts you, you can start to handle the babies from birth, but if she’s new or is a nervous mum, leave them 3 – 5 days, or more if she still exhibits stressful behaviour at that time. Start by gently stroking and moving the babies around in the nest, just to get your scent on them. See how mum reacts. If she’s fine, then the next day you can pick them up and cup them gently in your hands. Pinkies haven’t yet gained much control over their muscles, so they will spasm and pop all over the place. Be very careful not to drop them, and to only hold them inside the cage or over your lap so that if they do fall, they don’t fall far. If this does happen, don’t panic, pinkies (like human babies) usually bounce ;) It’s important to handle the bubs at least once a day, to make them nice and friendly. The more they’re used to being handled, the better pets they’ll make. I’ve found that the most vital time to handle them loads is the few days before and after they open their eyes – I think it impresses on them more if they see and feel you as the first thing they experience.
At a few days old, babies that will be dark (agouti, black, etc) will start to develop pigment. At 8-10 days they will have velvety peach fuzz (from here they are called fuzzies), and now you should be able to sex them by checking for nipples under their arms along their sides. Their eyes will open at 12-14 days, and by 3 weeks they should be nibbling on solid food and hopping around the cage. At this stage they enter into what’s known as flea or popcorn stage. If you haven’t handled them much, or if their parents don’t have fantastic temperaments, they will pop out of your hand and leap everywhere. I often find that with enough handling, they pretty much skip this stage – but you will need to be careful taking them out of the cage anyway, just in case! At 4 weeks, it’s time to separate the boys and girls. This is VERY important! If you don’t, at about 5 weeks of age, mum and all your female babies will be pregnant! If you’re lucky, you’ve kept track of your boys and girls from your nipple-sexing, if not, you’ll need to sex by genitals at this stage because nipples are now hidden in fur.