I found it. Ven Hsuan Hua's site has changed a lot and the list of documents seems shorter. I first read the Rahula story in a html file but after typing 'Rahula' as keyword and searching all the files, there was no match.
But there is a pdf file and that contains a reference to Rahula's gestation period.
The Shurangama Sutra Vol 4
"Rahula was the Buddha’s son, but he was not conceived through sexual intercourse.
Shakyamuni Buddha was married at seventeen years old and left
the home-life at nineteen. Although he married, it was not a sexual
relationship. Before the Buddha left home, Yashodhara wanted to
have a son by him. So the Buddha pointed to her belly and she
conceived. This may sound like a myth, but this is how it is actually
recorded in the Buddhist Sutras. You may want to figure out how
she could get pregnant just by having him point at her, but you’ll
find it’s an inconceivable and ineffable state of affairs.
Rahula’s name means “Obstacle.” Rahula lived in his mother’s
womb for six years. This is another case of cause and effect. In a
former life, Rahula had plugged up a mouse-hole, and it took six
days for the mouse to gnaw out another passageway. As a result,
Rahula had to undergo the retribution of dwelling six years in his
mother’s womb."
The first paragraph is new to me. But the second one is familiar alright, only it's a mouse hole not snake. Sorry, I'm very bad when it comes to noticing animals.
I guess this all goes to show there are more than one source for these biographical accounts of the lives of the Buddha and his disciples.
Cheers