When someone questions that statement, she says there are siblings who are 9 months or less apart in age and gives a link explaining. When someone points out that that link in particular is talking about rare cases of women having more than one uterus, she rebuts with "No, it also applies to women who mensturate throughout pregnancy."
Two of us try to let her know how the female body works when pregnant, and of course she back tracks a bit.
Not huge stupid, but certainly entertaining. Either that, or I'm simply bored.