Underweight & TTC.....

Anita • I`m 22 & have been TTC for 7 mon. now unfortunately I have irregular periods. I`ve been in a great and loving relationship with great partner for 5 years.
Ever since I can remember I've been underweight & growing up it never bothered me. But now that I'm TTC it seems never being going over 100 lbs is not the best thing. I'm 5'5 and at the moment weigh 97 lbs. It's super hard for me to keep on the pounds. Who else has the same problem? How do you gain weight and keep it on?