Women who are underweight or have a body mass index (BMI) that is below may want to consider gaining weight to maintain a healthy lifestyle. Being underweight can lead to many health issues for women such as weakened immune systems, decreased muscle mass, unhealthy hair, skin and nails, weakened bones and the inability to menstruate. Gaining weight and maintaining a healthy lifestyle can decrease the probability of these health issues. Women should look for healthy ways to gain weight, rather than gaining weight through increasing fat on their body. Start with Step 1 below for some helpful information on how to gain weight for women.