The famous prayer associated with Mary while she was carrying Jesus in her womb is called the Magnificat.
It's found in the Gospel of Luke (1:46-55) and is spoken by Mary upon visiting her cousin Elizabeth, who is also pregnant (with John the Baptist). Elizabeth greets Mary with the words, "Blessed are you among women, and blessed is the fruit of your womb!" and Mary responds with the Magnificat, a song of praise and thanksgiving to God.
Here's the text of the Magnificat from the New King James Version (NKJV) of the Bible:
46 And Mary said: “My soul magnifies the Lord,
47 And my spirit has rejoiced in God my Savior.
48 For He has regarded the lowly state of His maidservant;
For behold, henceforth all generations will call me blessed.
49 For He who is mighty has done great things for me,
And holy is His name.
50 And His mercy is on those who fear Him
From generation to generation.
51 He has shown strength with His arm;
He has scattered the proud in the imagination of their hearts.
52 He has put down the mighty from their thrones,
And exalted the lowly.
53 He has filled the hungry with good things,
And the rich He has sent away empty.
54 He has helped His servant Israel,
In remembrance of His mercy,
55 As He spoke to our fathers,
To Abraham and to his seed forever.”
The Magnificat is a powerful declaration of God's faithfulness, justice, and mercy. It's a central prayer in Christian liturgy and continues to inspire believers today.