The Rebbe's letters on this subject accompany medical care; they do not replace it. Every medical decision should be made with qualified doctors.
The Rebbe's guidance for couples hoping for children, and on adoption:
Medical and spiritual efforts together. Seek expert, up-to-date medical advice while keeping firm faith in G-d's blessing (IK 688, 6282); both natural efforts and spiritual actions are needed (IK 2531). Don't be frightened by medical opinions about pregnancy complications (IK 1576). Careful observance of family purity is central when hoping for children (IK 2177, 8687, 9168). On halachic questions about fertility procedures and testing, consult a halachic authority (IK 666, 1411, 5059, 5408); halachic observance should not be compromised because of medical advice (IK 5715).
Adoption. Adopting and raising a child in holiness and Torah values is considered by some a segulah for having children (IK 1616). Adoption should be only from a child of kosher Jewish lineage (IK 6252). When a child's maternal Jewish status is uncertain, an impartial rabbinic ruling must be sought (IK 8461, 10112). The Rebbe raised serious halachic and practical concerns about modern adoption practices (IK 8766, 8772, 9138), and urged communal awareness of these issues (IK 8954). Questions of disclosure and shidduchim for an adopted child should be brought to a Rav (IK 10085). For adoptive parents, positive peer influence and personal example are key to guiding the child (IK 10579).
See also: Health – Pregnancy and childbirth; Marriage – Family purity and mikveh; Health – Following the doctor.