... You state your heartfelt desire to be blessed with additional healthy and viable children:
... Understandably, in addition to the above [advice of going to a fertility specialist,] the two of you are to enhance your performance of Torah and mitzvos, particularly with regard to observing the laws and regulations of family purity, tznius, and the like, as well as increasing your donations to tzedakah. Pray to the Giver of Torah and mitzvos that He should fulfill your heartfelt request for additional children.
It would also be advisable for your husband's tefillin as well as the mezuzos in your home to be checked to assure that they are all kosher according to Jewish law. Also, observe the fine custom of upstanding Jewish women of giving tzedakah before lighting the candles every erev Shabbos and erev Yom Tov, for the sake of the fulfillment of your heart's desire.
It would seem from your letter that you are the daughter of a Kohen. If your husband is not a Kohen, then in addition to the above, it would be appropriate for your husband to be knowledgeable in the content of several tractates of the Talmud — they may be of the smaller tractates, such as Tractate Kallah and the like.
(Igros Kodesh, Vol. XIV, p. 383)
... Regarding that which you write about your wife — that since the birth of your first child you have not yet been blessed with additional children:
On the whole one must be steadfast in one's faith in G-d, the Creator of the world and the Essence of Goodness, that He will fulfill your heart's desire for healthy children.
Nevertheless, it would be proper for you to enhance your observance of the laws of family purity and modesty, though I trust that the basics are already being adhered to.
... See to it that you have unwavering bitachon in the Giver of the Torah and its commandments that in the near future G-d will fulfill your heart's desires.
(From a letter of the Rebbe, dated 8 Iyar, 5718)
Auspicious Deeds That Can Assist