Constipation in baby. I would start by asking if the baby is actually constipated. Often around two weeks babies (especially breast fed infants) don't stool as often. Many people think this means they are constipated. Some breast fed infants poop once a week! if the stool is soft when it comes out then the baby is fine. If the stool is hard (like rabbit pellets) then you should talk to your doctor.
Constipation. In a two week old, constipation is very common. A breast fed infant may go up to 7 days without stooling. As long as stool is soft, it is okay. Now, if a 2 week old is constipated, they can benefit from formula change, belly massage, pulling knees to abdomen, and if that does not work - speak to your pediatrician about use of karo syrup. A 2 week old should only receive formula or breastmilk.
Maybe prune juice. If using formula for feeding, a change of formula may solve the problem. Make sure it is mixed according to the label--not too little/much. If breastfeeding, u may try a teaspoon of prunejuce may help. No bm for 2-3 days is common and i would not worry much. If worsens, consult doc. Good luck (i misread your question before--i thought it was for a 2 yrs old and not two-week old, sorry about that).