Age of the egg (depend on personal preference)
17 days old (úp mề): Early-develop embryo, the chick is not old enough to show its beak. You just see a white-pinkish wrapped around the yolk.
19-21 days old (lộn): The chick inside is old enough to be recognizable as a baby duck and has bones (and some feather) that will be tender when cooked.People also eat ones that are not develop properly after 9-12 days (called trứng vữa). They taste more like hard boil egg with bigger and softer yolk.
Boil fetal duck egg (balut) for 30 minutes. It should be eaten while warm (hot). Use a shot glass to hold the egg. Crack shell with a teaspoon and sip the surrounding broth then peel and eat everything inside with salt, pepper and Vietnamese coriander leaves.