We had the exact same problem! Dd was so uncomfortable the first few weeks she cried all the time. We talked to our ped about it. He said that some babies have problems coordinating movement of their bowel muscles and their bum muscles. Our dd would push and push while clenching her bum tight! She also had lots of issues with gas pains.
What worked for us... First I eliminated all known gas-inducing foods from my diet - everything that Carrie listed plus beans, cabbage, and cauliflower, and on the advice of our ped, nuts (including peanuts) and shellfish. Eliminating dairy was key. We also gave her a dose of gas drops with each feeding (Mylicon is the brand name but generic works just as well and is much cheaper - we use Target brand).
We kept up my dietary restrictions until 3mos, at which time we started reintroducing things one by one. I still avoid nuts and shellfish. We continued the gas drops with every feeding until about 4mos. We tapered her down slowly - now at 6-1/2mos, she gets them twice a day.
Breastfed babies can go alot longer without poo. Dd has gone as long as ten days. I don't worry about her until she gets uncomfortable. Then we try warm baths and gentle belly massage. Our ped also rec'd taking a rectal temp to move things along, but that never really worked for us. Until she was about 4mos, though, she pretty much only pooed while eating - that was when she was most relaxed.
Definitely try the diet changes and the gas drops. If that doesn't work, our ped also rec'd Baby's Bliss Gripe Water. It is available online and at most natural foods stores.
Post back and let us know if things get better.
