Your puppy will go through several stages of growth before adulthood, including an especially rapid phase right after birth and an awkward lanky phase just before reaching adulthood. If he starts to look too thin or in any way unhealthy, schedule a visit to your veterinarian.
In general, if your puppy is at a healthy weight his ribs should not be plainly visible. He should be gaining weight at a steady pace, doubling his mass in the first week of life and reaching about one-half of his estimated adult weight by around 4 months. A healthy puppy is rarely overweight, but if his ribs show then he is likely underweight. You should be able to feel his ribs easily with your hands, but they should not obvious to the eye.
As your puppy goes through stages of growth, he may appear thin or heavy at times, depending on his breed and growth pattern. To determine whether his weight his healthy, assess his overall body condition. Look at him from the side and the top, and feel his body to determine the condition. A puppy who is too thin will have visible ribs, protruding hip bones, a tightly tucked waist and a visible spine. At an ideal weight, your puppy's ribs won't be visible but can be felt easily, and he will have a defined waist that gently curves inward when viewed from above and is lightly tucked up when viewed from the side. A layer of spongy fat over the ribs and the absence of a waist indicates that your puppy is overweight.
Some breeds have very little body fat and naturally slender bodies, especially the sight hounds, like the greyhound and Saluki. It may be normal for puppies of these breeds to have some visible ribs. For example, the last two ribs on a healthy greyhound will often be visible; a dog of the breed is considered too thin when more than two ribs show or his hip bones become prominent. Always check with your veterinarian if you are concerned about your puppy's weight, but keep in mind that If your puppy is one of the breeds that is naturally lean, it may be okay for the last ribs to be slightly visible.
Poor nutrition or a lack of food can cause a puppy to be too thin, leaving his ribs showing. If your puppy is receiving plenty of nutritious food and is still skinny, he may have a health problem. Intestinal parasites, contagious diseases like parvovirus, bacterial infection, diarrhea, vomiting and intestinal blockage can cause a puppy to be too thin. Your veterinarian may need to run urine, fecal or blood tests to determine the reason your puppy is not at a healthy weight if a poor diet is not the cause.