<s>[INST] my son was just born on March 9th, so he is almost 7 weeks old. He seemed to have a birth mark on his elbow. Today, my wife was putting a short sleeved shirt on him and noticed that under the mark there felt like what is a small lump. There is not a lump present on the other arm, under the other elbow. It does not seem to cause him discomfort. What could this be and what should the next course of action be? [/INST] Thank you for posting your question.Birth marks in children usually evolve over time and are all not necessarily present at birth. The mark under the elbow that you mentioned might have evolved to become thicker and hence palpable.However it is important to know what kind of birth mark ( hyper pigmented/depigmented/hairy etc.) that you are talking about, which could help us rule out any other cause.By and large it seems to be a benign occurance and the only thing that needs to be done is keeping a watch for similar lumps on other parts of body or an increase in size of the existing lump. If so, a medical consultation would be advisable.Also rule out some trivial local injury that may have caused a temporary swelling under the elbow.If you need any more advice regarding this, do write back. </s>