On the Expression "Mazal Tov"

Having just congratulated a friend on the birth of a son, I suddenly became curious on the origin of the expression "Mazal Tov". A quick search in Google, found that the origin of the phrase is from Rashi who uses the expression on his commentary on Genesis 30:11

י  וַתֵּלֶד, זִלְפָּה שִׁפְחַת לֵאָה--לְיַעֲקֹב בֵּן.	10 And Zilpah Leah's handmaid bore Jacob a son.
יא  וַתֹּאמֶר לֵאָה, בגד (בָּא גָד); וַתִּקְרָא אֶת-שְׁמוֹ, גָּד.	11 And Leah said: 'Fortune is come!' And she called his name Gad.

“Luck has come”: Heb. בָּא גָּד. Good luck has come [Targum Jonathan ben Uzziel] similar to (Shab. 67b)“ May my fate be lucky (גָד גַדִּי) and not fatigued,” and similar to this (Isa. 65:11), “who set a table for Gad.” According to the Aggadah [Midrash Aggadah in the name of“some say”], he was born circumcised גָּד, meaning “cut off”), like Dan. 4:11,“cut down (גֹּדוּ) the tree,” ..

בא גד: בא מזל טוב, כמו (שבת סז ב) גד גדי וסנוק לא, ודומה לו (ישעיה סה יא) העורכים לגד שלחן. ומדרש אגדה שנולד מהול, כמו (דניאל ד כ) גדו אילנא..

The original meaning of the word Mazal - is a constellation of stars, which astrologists believed would influence your life.

See also this article [Hebrew] on how Sephradic (specifically Iraqi) communities would use "Mazal Tov" only for male children, and "Siman Tov" for females.