a) Seven month children would indicate resistence and longevity. The Midrash Rabbah 14:2 states:
בעון קמיה דרבי אבהו: מנין שהנוצר לשבעה חי? אמר להון: מדידכון, אנא
ממטי לכון, זיט"א אפט"א, איט"א אוכט"א:
They [some Greeks] asked R. Abahu: From where do you know that a fetus
formed at 7 months can live? He replied: From
your own [language] I will prove it to you: zeta epta, eta octo [this is a wordplay on "zeta — seven, eta — eight" also meaning something like "fetuses of seven months are more likely to survive than those of eight" ("Live, seven! Dying eight")].
Therefore the seven month baby is not premature but a product of an alternative, faster maturation tract.
b) Indicates a sign of greatness and prophecy. That would explain why Midrash HaGadol on Shemos 2:2 states:
שכל הנביאים לא נולדו אלא לשבעה חדשים
“all prophets were born at seven months"