Thank you for your query.
I think you need evaluation for thyroid
function and anaemia
before planning for baby .You can take a multivitamin
syrup and healthy diet till then but actual root cause needs to be found out for you being underweight
I think you should see a physician for this problem and get yourself evaluated.
Hope you are satisfied with the answer.
If you have any further queries feel free to ask.
Wish you good health.