i sent 
1. letter from GYN confirming my pregnancy
2. wrote letter to embassy, asking emotional support from my mother during that hard time lol..
thats all

Hey Biology

DO NOT  mention pregnancy case at all !!! period. you are better off not sending any invitation letter than mentioning pregnancy case. Here is why

1) Daughters pregnancy case by itself is not a primary reason good enough to issue VISITOR VISA to your mom. The primary purpose of Visitor Visa is something different

2) The Consulates in Embassy DO understand the cultural differences among countries especially difference in western culture & asian culture. The notion of JOINT FAMILY in our countries & western countries is really really different. Basically, if your mom comes to visit you during pregnancy, she is working part time /full time as a baby care taker & visitors are not allowed to work. ( Whether its paid or not, It is considered as JOB). Its basically considered that your mom is trading one person's job as nanny housekeepr or baby care people, whatever you may call it.

My Suggestion: Dont mention that in letter or in interview, however, if they specifically ask your mom during intereview, tell em' the truth, DO NOT LIE  !!!

Sources: Personal experiences & many immgration forum !!!

soul survivor is right. Don't mention about pregnancy, simply state that your mom wanna visit you and take a tour around USA.

Tell the truth; reason for the visit is because you need them during pregnancy.
Looking after a pregnant daughter is as natural as raising the new born daughter. It is not work but a responsibility and US consulate who issues the visa is aware of this culture.

Illegally Working part or full time