Capturing for posterity my scribblings from a mailing list discussion:
The “You can’t have 2 women deliver a baby in 9 months” metaphor always bugged me, because there’s no room for changing scope in that (You can’t opt to just birth the torso in 5 months, then add the head and limbs at a later date).
My preferred metaphor for explaining the mythical man month is that of getting a piano up a narrow stairwell. One person trying on their own would take ages, and there’s a high risk of failure. Two or three can probably get it done reasonably quickly. But if you try to get eighty people to do it, they’ll spend ages arguing about how to go about it, get in each others’ way, and in the end, two or three of the more productive ones will probably do most of the heavy lifting while the others stand around.
I usually illustrate this point with a shot of Laurel & Hardy from their 1932 classic The Music Box
(A quick Google shows that I nicked this metaphor from this blog comment.)