Apparently all the women in the study were following there cycles and it worked out that when they conceived a lot of the time it was when the sun and moon were in the same degrees than at birth. I beleive there are lots of ways to determine twins. 10% of women ovulate twice a month and it happens that way sometimes and there are lots of theories related to gemini and 5th house.
Thats exciting that yours came up with twins. I will have to have a look