Well I don't have any hard-and-fast answers as to how exactly steroid use might stop us getting pregnant (except for what ladies on here and the Pred thread have found to be the case, and what I feel has happened to my own body), but I just did a very quick search and found two hits straight off, both from body-building websites, where obviously steroid use is much more common. On one there was this post:
A woman's reproductive system is in the line of fire when steroids are in use because of diminished estrogen activity. Her hypothalamis-pituitary-gonadal axis is off balance. With the increase of testosterone circulating, a decline in estrogen and progesterone occurs. Steroid use can result in the inhibition of follicle formation, ovulation, and irregularities of menstrual cycle which can lead to amenorrhea (absence or suppression of the menstrual cycle). A study conducted with 9 female weight-lifters showed all, but 2 women had menstrual abnormalities (Malarkey et al.; 1991).
And another, the website is pregnancy.lovetoknow.com/wiki/Low_Fertility_Caused_by_Steroids , and a quote from that page says:
Women who take regular corticosteroids and are hoping to conceive must be carefully managed by their physician for a weaning off period to avoid the risk of pregnancy problems and potential birth defects. For some women, the damage may be long-lasting as steroid use can have a significant impact on the menstrual cycle and the ability to conceive.
Now this was only the first two hits I looked at - I don't have time right now to do a better search - but it seems to me that there are definite known issues about steroids and female fertility. I know for us the use of steroids may be unavoidable as a way to avoid miscarriage, but if there is a possible disruption to the menstrual cycle/ability to conceive, this should be born in mind.
Just my tuppence worth, I am not a doctor!