She had several miscarriages. The Rebbe suggests strengthening her faith that Hashem will bring good

Be careful with the laws of family purity and modesty. Consult a specialist and avoid lifting heavy loads or running. Don’t inform anyone (except close relatives) about the pregnancy until the fifth month. Check the mezuzos. Let me know the good news. https://www.torahrecordings.com/rebbe/igroskodesh/011/010/3665