Rat Infant Mortality Rate

[hhw]

Yesterday, one of my female rats had a litter of 11. Upon inspection, 3 of them were dead. Now, since I'm under the impression that female rats will chew off the umbilical cords of the young and if the babies don't squeak and squeal when that happens (which a live one would), the female will just keep on and eat the dead baby. Since those 3 were intact without umbilical cords or any placental mess, I assume that they were born alive and died sometime after. Today, I found a 4th baby dead.

Now, initially I had the pair in together (female and the male), but I removed the male when I discovered the babies. I assumed that that may have been the reason the first 3 died (by pestering the female or whatnot), but that would not explain the fourth.

So, my question is is it natural for some of the babies to die off like this the first day or two due to not being able to compete for milk, or am I doing something wrong husbandry wise here?