"Ŝia bebo ofte metas siajn fingrojn en la buŝon."

Translation: Her baby often puts its fingers into its mouth.

[Mingan8]

How common is the use of definite article when referring to one's own parts of body? Would it be wrong to say en sian buŝon? Why not la fingrojn en sian buŝon?

[vikungen]

It is very common to use "la" instead, just like it is in many European languages.

Mia buŝo = la buŝo

In my native language Norwegian it would be the same:

Hun stikker fingrene i munnen (She puts the fingers in the mouth)

It is only if it needs to be specified for some reason that you would use a pronoun here instead of the definite article, same with Esperanto.

[ronstrelecki]

It is frustrating to learn something and then have it immediately tossed for something else arbitrary.

[FreddyBGood]

Agree with ronstrelecki, suggest translation would be: "Her baby often puts its fingers in the mouth." If we want to say "in its own mouth" we would say "en sian buŝon"

[PurpleHuedMagPie]

Try making it someone's else's mouth like a father. Then it shows the structure of the original sentence. It goes from "en la buŝo" to "en la buŝo de ĝia patro." That's how I understand it.

[FreddyBGood]

Mi komprenas la reĝulon, mi jus sentas ke ĝi estas pli rekta, pli akurata kaj pli facila diri sian buŝon. Ne ambiguaĵo. (Dankon pro vian klarigon; ĝi helpas pensi pri ĝin.)

[PurpleHuedMagPie]

Nedankinde