My friends and I were talking about this last weekend. I don't think you can really figure them out until they are at least a year old. As soon as you spot a routine- a growth spurt, teeth, cold, drs apt, sleep regression, diaper rash, etc- messes it back up.
But, to answer your question, 6 weeks is about the time of the second major growth spurt. After that is the two month shots, the three month growth spurt and sleep regression, then the four month shots, then solids....
It does get easier as you gain in experience, though, and as you get to know your little one's personality better and can communicate better.
ETA: Sorry! DS hit the enter key on me. lol. But it will get better!