Play a recording of a three-day-old French baby crying, and one of a three-day-old German baby, and researchers can tell them apart. The babies have never spoken a word.
In a 2009 study, sixty newborns were recorded at three to five days old. French babies cried with a rising melody. German babies cried with a falling one — each matching the intonation of the language spoken around them.
The explanation is in the womb. Low-frequency sound passes through the womb wall far better than words do, so in the last trimester a foetus hears no vocabulary at all. It hears melody, rhythm and stress, and it arrives already tuned to them.
Which means the useful thing for a parent is nothing technical. Talk naturally. Sing, even badly. Read aloud long before it seems early. They may not understand the words yet, and they are already learning the music of your language.
The first thing we learn isn't words. It's music.