Final answer:
The double bubble sign on an X-ray in a newborn with Down's syndrome typically indicates an intestinal obstruction, which is often due to duodenal atresia.
Step-by-step explanation:
The significance of the double bubble sign on an X-ray in a newborn with Down's syndrome is that it indicates the presence of an intestinal obstruction. Specifically, this is commonly associated with duodenal atresia, a congenital condition where the first part of the small bowel (duodenum) has not developed properly. It is not a normal finding, nor does it suggest a diaphragmatic hernia or pneumothorax. Babies with Down's syndrome are more prone to certain congenital malformations, including gastrointestinal ones like duodenal atresia.