My understanding is that B6 helps regulate ovulation somewhat so that your egg ruptures the follicle when it's mature, but not overmature, so that your corpus luteum is stronger and will produce more progesterone. My understanding is that progesterone is really what determines luteal phase length, and B6 does boost progesterone levels, but indirectly, by helping the health of your follicle and CL... at the very least, this cycle you should be able to go in for a progesterone test as well. Keep in mind though, that they mainly use the test to confirm ovulation, not to assess if your progesterone level is high enough to sustain a pregnancy. According to paper I read, progesterone is secreted in waves or pulses, so it might be high, and then a few hours later, it might be much lower. So it's not really reliable. Also, some doctors believe in LPD as a fertility issue, and others do not. My doctor does not feel that a short LP (or spotting, in my case)has any bearing on our subfertility... To that end - as you can see from the B6 thread, there have been a number of BFPs among ladies with shorter LPs...