http://revach.net/article.php?id=2542

Section: Halacha   Category: Tshuvos
Rav Moshe Shternbuch: Do You Make A Shalom Zachor If The Baby Is Not There?
Rav Moshe Shternbuch (2:205) says that the Rema (YD 265:12) says there is a Minhag to make a Shalom Zachor. There are a number of reasons given for this minhag.  The Trumas HaDeshen (269) says that this minhag is based on the gemara in Bava Kama (80a) where Rav, Shmuel, and Rav Asi went to a "Yeshua HaBen".  Rabbeinu Tam says this was a Seudah to celebrate the birth of a baby boy.  The Trumas HaDeshen says this was a seuda to give thanks to Hashem and is held on Friday night because that is when people have time to attend.   
 
The Taz says that there is a Mekor in the Medrash that says, no Korban is brought unless it first went through a Shabbos. Similarly he says, there can be no Bris without a Shabbos and therefore on Shabbos we make a party.  According to these two sources it is not necessary for the baby to be present.
 
The Drisha says that we make a Shalom Zachor to console the baby on the loss of his Torah that he learned in the womb.  We eat Arbis as a form of Aveilus.  According to the Drisha the Shalom Zachor would only be held if the baby were there.
 
However says Rav Shternbuch, the main reason for a Shalom Zachor is in accordance with the Zohar that says the whole world receives its Bracha from Shabbos.  On Shabbos the Bracha comes down and is given out to each weekday in turn.  Therefore on the Shabbos before a Bris all the people come together to give the father of the child brachos and they rejoice in the Bracha that has come down because of the upcoming Bris.  Therefore he concludes that the Shalom Zachor should be made whether the baby is there or not.
 
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.