bemtsa hadaf neemar she haman haya maguid lahem leyisrael i ben 9
larishon ve`i ben 7 laacharon....
nish`elet hasheela lo hayu mamtinim 3 chodashim ben haguirushim la irussim hasheniim kedei levadot shel mi haubar.( she haben lo yikashel bechashash mamzerut...)
(Translation: The Gemara says that the Man would reveal whether a divorced woman's baby was the son of the first husband (and was born full term, after nine months), or was the son of the second husband (and was born prematurely, after seven months). What about the requirement for a woman to wait three months after divorcing before marrying a second husband?)
That requirement is a later rabbinical enactment, which did not exist in the Midbar. Furthermore, perhaps it did not exist in the Midbar because it was not necessary -- the Manna solved the problem that the Gezeirah was intended to solve!
Alternatively, even if that Gezeirah did exist in the Midbar, perhaps the Man was needed for those who transgressed the Gezeirah.
All the best,
Y. Shaw
(Ha'chiyuv l'hamtin 3 chodashim hu rak chiyuv d'Rabanan, she'od lo tiknu b'zman ha'Midbar. V'Ulai zeh ha'sibah she'lo tiknu -- mishum she'lo hayu tzrichim l'gezeirah zu, mishim sh'hayah sham ha'Man!
V'od yesh lomar, sh'afilu im haysah sham ha'Gezeirah, hayah tzorech l'ha'Man bishvil ha'nashim sh'avru al ha'Gezeirah.
Kol Tuv,
Y. Shaw )