Gestation is normally 22 days, however this can vary from 21 to 23 days and, in rare cases, up to 26 days. Some moms start showing up to a week before delivery and some only show a day or two ahead of time. They will begin to take on a definite pear shape. Some pregnant and nursing moms can have a change in personality and become aggressive, even though they've never shown signs of aggression before. This is normal and they will revert to their original personality after weaning, or in some cases even while they are still nursing, if they have a strong bond to you in the first place. Pregnant rats should have an increase in protein in their diets. They should also be moved to a maternity tank a couple of days before they are due and given nesting material so that they can prepare their nest for their babies. I use an aquarium for my litters because it's warmer than a cage and there aren't any shelves. Mom could be tempted to carry her babies up and down the shelves, accidentally causing injury. I usually use paper towels for nesting material because they are easy to change when they get soiled. Some moms won't build their nest until after the babies are actually born, and this is also normal.
When mom goes into labor, she will usually become very still and curl up in a corner of her tank. Then, she will suck in her sides while stretching her back feet out behind her as she pushes the babies into position to be born. It normally takes 1 to 2 hours for labor and delivery.
When the babies start being born, mom will help pull them out. She will also cut each umbilical cord with her teeth and eat the placenta from each baby. She will clean each baby until it begins squeaking and wiggling. She will also step on the babies and generally manhandle them. Do not be alarmed, this is normal behavior.
Do not house males with a mom and her new babies because females come into heat within 24 hours of delivery. Many breeders house two new moms and babies in the same tank without any problems. I choose to house new moms and their babies separately. They MAY try to steal each other's babies, accidentally causing injury. Also, if the babies were born a day or two apart, it will be difficult to determine which babies belong to which mom. This would cause problems with pedigree information. If you do choose to house them all in the same tank, be sure to watch for any difficulties.
New moms know exactly what to do to care for their babies. However, you can help by making sure they get plenty of protein and fluids, occasionally putting a heating pad set on LOW under half of their tank, and rebuilding mom's nest for her when you clean the tank. Also, if she has a very large litter (15 to 20 babies), rotating the babies is a good idea. Otherwise, they may not all get to eat. You can tell when they have eaten because they will have a yellow line on their abdomen. It is very obvious. No yellow line means they have not eaten. Be sure to always leave her with groups of at least 5 or 6 babies, or she may not take the time to feed them. After the first week, the babies should be strong enough and you can stop the rotation. And, this is only necessary in very large litters. Also, nursing moms may have loose stools.
I always give new moms time away from her babies each day. It's a good idea to have her play with her regular cagemates each day so you won't have to go through reintroductions when the babies wean and mom moves back into her normal cage. Be sure the babies are warm and protected while she is away. The first couple of days, she should only be away from them for short periods of time. But, after the first week, she can be allowed to play away from the babies for an hour or so each time. She will appreciate the break! The babies will wean in 4 to 5 weeks and males should be separated from their mother and sisters at that time.
SEXING BABY RATS
Here is an article that will help you sex the babies:
How to Sex Rats, by Lucie Mann
Page 3: When to Call the Vet / Orphaned Babies