The first time my informant got her period, her mother told her to go to the stairs, hold her breath, and walk down the number of steps she wanted her period to last for. For example, my informant decided that she wanted her periods from then on to last for three days, so she went down three steps while holding her breath. According to my informant, it worked for her; her periods now last three days.
I asked her why she didn’t just go down one step, and she said, “because it wouldn’t be possible, biologically, so to keep the legend true, you have to go down at least three or four.” This response suggests that there’s an element of conscious self-delusion for every girl who performs this custom, and that the belief is important more for its own sake than for the fact that it works.
My informant proposed that going down the stairs represents that the performer is taking the steps to becoming a woman. The girl holds her breath because Buddhism (the main religion of Thailand) encourages believers to endure suffering. Not breathing also symbolizes the pain of menstruation.
I agree with her assessment. A girl’s first menstruation is, biologically, the marker of her transformation from girl to woman. Taking physical steps represents that she is crossing that threshold.
Annotation: This folk custom appears in the 2001 Thai movie The Legend of Suriyothai. http://www.imdb.com/title/tt0290879/