Should I tell my husband when our daughter gets her first period, or it is irrelevant?

Your daughters health is never irrelevant to the knowledge of your husband. You should be comfortable enough to discuss all things family and life related, including the health of your children, your future, your finances and more. Based on a healthy and open relationship, the better the communication the stronger the foundation of trust. Trust and allow that your husband is able to understand the changes your daughter will go through all stages of her life, so that he can be there for her, for you, for his family.