Shaikh Abdul Aziz Ibn Baz rahimahullah said:

“The Sunnah is to shave the head of the boy when naming him on the seventh day only. As for the girl then her hair is not shaved due to the statement of the Prophet sallahu alayhi wa salam:

كل غلام مرتهن بعقيقته تذبح عند يوم سابعه ويحلق ويسمى

“Every boy is mortgaged by his ‘Aqiqah, so slaughter for him on the seventh day, and shave his head, and name him.

[Reported by Imam Ahmad and the Companions of the Four Sunan and its chain is Hasan]…”

[Taken from here]

Also Shaikh Ibn ul-Uthaymeen rahimahullah said the same:

“Shaving her head on the seventh day is not Sunnah for her like it is Sunnah to shave the head of the boy. As for shaving it for a benefit which the questioner mentioned [i.e. to allow her hair to grow long], then if it is correct the people of knowledge say: shaving her hair is disliked. However, if it said that it is affirmed that this shaving is from the ways to grow the hair and make it plentiful, then their is no harm in this because it is known that something disliked is no longer disliked for a need”.

[Taken from here]

However, the majority of the Scholars of Sunnah believe it is Sunnah for the newborn girl as well.

Imam Abul-Hasan as-Sindhi rahimahullah, one the great commentators on Hadith said commenting on the Hadith:

Every boy is mortgaged by his ‘Aqiqah, so slaughter for him on the seventh day, and shave his head, and name him.

He said: ‘Every child: that is: newborn male or female…”

[Taken from here]

Shaikh Al-Albani rahimahullah was asked whether the hair of the newborn girl should be shaved?

He said: “[Yes], like the boy”

[Taken from here]

Imam San’aani rahimahullah said:

“His sallahu alayhi wa salam statement in the Hadith of Samurah [Shave his head] is evidence of the legislation of shaving the head of the newborn on the seventh day. What is apparent is it is general for shaving the hair of the young boy and girl”.
[Taken from here]

So the majority of the Scholars of the Sunnah are of the opinion that the parents shave the hair of the newborn girl. Although there is a difference of opinion, the Hadith quoted above is general and includes the newborn girl.

Allah Knows best.

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan