Relatives: I'm Pregnant and I Did Invite My Mother from Ukraine to Usa, Visa Was Denied,

Hi, my name is roxy. I and my husband we are legally (green card) in usa since 2012. I got pregnant, my due day is in may 2016 and i really want my mother to come for my delivery day and help me fist few week. We did an invitation and letter from my doctor to confirm the pregnancy. The interview had a negative answer. I don't understand why?!
There is anything i can do for it? The is any law? Do i have any right to call some parent to help me with my child? Please, help!!! I'm really upset, this was the 4th time in 4 years she tried to come for visiting.
Thanks.
Roxy

Relatives: I'm Pregnant and I Did Invite My Mother from Ukraine to Usa, Visa Was Denied,