Is it wrong to prefer a baby boy over a girl?

Yes, it is wrong to value a son above a daughter. Hoping for a boy is a natural feeling and not a sin in itself, but Islam condemns being upset at the birth of a girl and treating daughters as less important. Both sons and daughters are a gift from Allah.

Boys and girls are both Allah's gift

The Quran reminds us that the gender of a child is entirely Allah's decision:

لِّلَّهِ مُلْكُ ٱلسَّمَـٰوَٰتِ وَٱلْأَرْضِ ۚ يَخْلُقُ مَا يَشَآءُ ۚ يَهَبُ لِمَن يَشَآءُ إِنَـٰثًا وَيَهَبُ لِمَن يَشَآءُ ٱلذُّكُورَ "To Allah [alone] belongs the kingdom of the heavens and the earth. He creates whatever He wills. He blesses whoever He wills with daughters, and blesses whoever He wills with sons." Quran 42:49

Notice that daughters are mentioned first. The next verse says He gives both to some and leaves others without children (Quran 42:50). Neither parent controls the outcome, so no mother should ever be blamed for having a girl. For the rewards that come with raising daughters, see What is the reward for raising daughters in Islam?

What the Quran condemns

Before Islam, some Arabs felt shame when a girl was born. The Quran describes this: "Whenever one of them is given the good news of a baby girl, his face grows gloomy, as he suppresses his rage" (Quran 16:58). The next verse calls their judgment evil (Quran 16:59). Disappointment that turns into shame, anger or neglect of a daughter belongs to the old way of thinking, not to Islam.

A feeling versus an action

A passing wish is human. Many parents hope for a certain gender, such as a boy first, a girl to balance the family or the other way around. A wish alone is not a sin.
What counts is how you treat your child. Love, time, education, food and opportunities should be equal.
Check your reactions. If you feel disappointed, make dua, ask Allah for contentment, and remember that He knows what is best.

Treating children unequally also causes harm. See Is it haram to favor one child over another?

What the Prophet ﷺ said about daughters

The Prophet ﷺ said: "Whoever has three daughters, or three sisters, or two daughters, or two sisters, and he keeps good company with them and fears Allah regarding them, then Paradise is for him" (Jami at-Tirmidhi 1916). In another report, he said that those who are tested with daughters and treat them well will have them as a shield from the Fire (Sahih al-Bukhari 5995). Daughters are described as a blessing and a path to reward.

If family or culture pushes for a boy

Some families, or cultures, still put pressure on couples to have a son. Respond with kindness and firmness: "We're grateful for whatever Allah gives us." Don't let a relative blame the mother, and avoid gender-based comments such as "Oh, another girl." If you're celebrating a birth, treat a girl's arrival with the same joy. See Can Muslims have a gender reveal party?

Key points

Allah decides whether a child is a boy or a girl.
The Quran condemns gloom at a girl's birth.
A passing wish is human; unequal treatment is wrong.
The Prophet ﷺ promised great reward for honoring daughters.

Tip: If you feel disappointed at an ultrasound, take time to make dua and talk with your spouse. The feeling usually fades once you hold your baby.