It depends on how much the baby weighed at birth. And it depends if the baby was breast-fed, the average weight gain of an infant is 6 oz a week on breast milk.
Ask the doctor
All babies are different. My son is 5 weeks old and weights 12pounds 9 ounces (5.7kilos) but he is a very big boy. If you have concerns abut your babies weight its best to check with your doctor or health care provider.