Announcing the Molad prior to Birchas Hachodesh:
A. The law:
It is proper for one to know the Molad prior to reciting the blessing over the new month, [and so is the Chabad custom, as writes Admur in the Siddur]. [One is to know the exact time of the Molad, including the day, hour, and Chalakim.]
Other opinions and customs: Some write it is improper to announce the Molad to the community. This is hinted to in the verse “Vein Esther Magedes Moladita.” The Sephardi custom is not to announce it.
Who announces it? It is customary in all Shuls for the Chazan or Gabaiy to announce the Molad prior to reciting the Birchas Hachodesh.
If one did not hear it: If one did not hear the Molad or does not know its date, he is to continue as usual with the Birchas Hachodesh.
B. The start of the custom:
The custom of knowing the Molad prior to Kiddush Hachodesh is first brought in the Siddur Shaar Hashamayim of the Shlah and is likewise recorded by Admur in his Siddur.
C. The reason behind the custom:
Reminiscent of previous times: The reason for the above custom is because in times that the month was sanctified based on testimony the witness would give the testimony of the moon and then a blessing over the month was recited. Thus, likewise today, we first recite the Molad, which is similar to the testimony, and then bless the month. Alternatively, the reason is because certainly the Beis Din would not sanctify the new moon without knowing the Molad beforehand, and hence we do the same today.
Chodesh means renewal: Alternatively, the reason is because the entire meaning of the word Chodesh is from the renewal of the new moon, and it is we hence mention it during this time.
Tikkun Pegam Habris: The knowing of the Molad during this time is auspicious for the Tikkun of Pgam Habris.
 Siddur Admur; Siddur Shlah; Shaareiy Efraim 10:37 “Some are accustomed”; Shaareiy Rachamim 10:47; Aruch Hashulchan 417:8; Pischeiy Olam 417:1; Kaf Hachaim 417:8; Karnos Tzadik 3:93; See Shaar Hakolel 26:7; Yaskil Avdi 7:27-4
 The exact time that the new moon will appear in Jerusalem.
 Shaareiy Rachamim 10:47
 See Shaareiy Rachamim 10:47 in name of Likkutei Maharich; Oheiv Yisrael Parshas Bo
 So is the custom of Anash; See Shaarei Efraim 10:36
 Shaareiy Efraim 10:37
 See Miseches Sofrim 19:9
 Shaar Hakolel 26:7
 Aruch Hashulchan 417:8
 Shaareiy Efraim 10:37
 Shaareiy Rachamim [in Shaareiy Efraim] 10:47