Rat Breeding

Question
Alright.. I plan on breeding rats so that I have snake food at a lower cost. I
know many people do not believe in breeding rats for food, I personally dont
see a problem with it. I want to do this the most ethical way possible and
don't want to put mom's body through too much stress. She will not be bred
constantly. She will have time to nurse her litter for a couple weeks before the
male in reintroduced to her. I will be periodically picking off babies from the
litter for food, but not all at once. Mom is very used to me. We got her from a
local breeder when she was about 3 months old. We have had her a little over
a month so she is going on 5 months.
For the last few weeks we have just left her in the cage with the male we
purchased.
We just removed him because we do not want her to get pregnant again after
she has birth.
I have a few questions.
She doesnt appear pregnant. There is no noticeable  weight gain, but her
stomach is taught.
I have done some research but I really have no idea to tell if she is for sure
pregnant. What is the likelihood she is pregnant if she has been with a male
for 3-4 weeks?
Also, Being that they are handled regularly and tend to seek attention, is it
likely that dad would try and eat the babies if reintroduced to the cage or if
he happened to be here during the birth?
I actually care about the adult rats and they are very well taken care of, rats
here just go for $3 a piece and I have many other mouths to feed.
Please give me some info so I can do this correctly and without making my
rats miserable.  

Answer
 First of all, I'm glad you've decided to do things the right way instead of not caring about your rats at all.  Rats are usually pretty good breeders and if you leave them with a male for around 2 weeks they are usually pregnant, though if they don't get along or the male isn't interested it may take longer.  Rats often don't show any sign of pregnancy until the last week, sometimes not even until the last few days and if the litter is small they may not even look pregnant at all.  If you don't want a back-to-back litter (which isn't advisable if you want to do things ethically) it is best to keep the female separated from the male until you are sure she is not pregnant.  A good way to tell if she is pregnant is to purchase a gram scale and weigh her daily, charting any weight gain.  Incremental weight gain can mean pregnancy.  The other signs are nest building, loss of hair around the nipples, and abdominal swelling.

 Usually father rats are pretty good with babies, but if you don't want a second litter right away it's best to just keep him away entirely.  Females come into heat every 5 days or so and the mother will very quickly get pregnant again, and he may also impregnate his daughters once they are around 6 weeks old.  If he happened to be there during the birth he probably wouldn't cause any problems, but things can happen.

 I don't know how many rats you are needing for food, but one mother may not be enough if you want to do things ethically.  A mother rat should be allowed one month's resting period between the weaning of her previous litter and being placed with a male to breed again, meaning you will have almost a two month period with no babies being produced.  Usually what most feeder breeders find effective is a harem method, meaning you have one male and several females that get bred in succession so there is always enough rats but the females still get a decent resting period.

 Well I hope that answers your questions, good luck!