Gererally speaking, there are three reasons can lead to dark circles. As we know, the skin of our eyes are much more thinner than other parts. so is baby. So it is blood vessels under the skin which cause dark circles. In fact, this situation is quite normal and many babies have such appearance. In addition, your baby's skin may be much more thinner than others, so it makes more obvious. and if your baby does not get enough sleep, this situation will get worser. Here goes the second reason, which I believe you should pay more attention. Nasal congestion can also cause dark circles under baby's eyes. This combines to allergies. When allergies happens, it always accompany with a runny nose, red or watery eyes, ear infections and dark circles. So if this happen, you should pay more attention to potential allergen, like dust, pets, mold and pollen.