Not that I've ever seen a formal study on it, but essentially you are talking about two different sources of hormones that dont' really interact. However the effects of the two are in conflict. That is why most women (e.g. competitors) will go off any hormonal B/C while cycling & in contest prep because the B/C introduces and enforces estrogen-driven fat depositing & probably a degree of water retention, etc. By removing this effect, you are removing one extra source of fat / water while trying to reach a contest or personal goal of LESS fat & water. The AAS, beign testosterone, is goign to be geared towards building muscle & reducing bodyfat. This is direct conflict w/ the goal of a B/C. So again to simplify the whole effort & get the most out of the cycle, most people remove the B/C because its results conflict w/ the goals of the AAS. Some women are able to cycle just fine while on B/C, using some very low dose estrogen/progesterone combination that works well w/ their body chemistry & doesn't introduce significant bodyfat or water retention to the equation.
That said, let's not forget that AAS will often "interrupt" a menstrual cycle, aka ammenorrhea ( geeeh...spelling..) So it may seem like your period is gone during the cycle, but you may still experience symptoms like cramping, bloating, etc. But the actual flow seems to disappear. It also occurs when bodyfat drops below around 12%, also an effect of going on a competitoin diet, training, etc. So the point here is that just because your period stopped doesn't mean that you can't get pregnant. I would even venture that just as B/C can become less effective while you are on anti-biotics, there's probably some effect w/ AAS as well. But just the loss of period can sometimes make the uninformed female cycler think that she's free & clear to not get pregnant either. WRONG. NEVER ASSUME! Just as if you are on an anti-biotic, I would definitely go to a back-up method such as a condom while you are cycling.
And to Daisy's point, while you are on AAS is the time you DON"T want to get pregnant either.
Give it at least 6 months for all the stuff to get out of her system. Its also been suggested give it that long to let your body's hormones rebalance after the cycle.
Just a quick quote from Anabolics 2000, page 13, Section on Steriod Side Defects -
Birth Defects:
Anabolic/androgenic steroids can have a very pronounced impact on the development of an unborn fetus. Adrenal Genital Syndrome in particular is a very disturbing occurrence, in which a female fetus can develop male-like reproductive organs. Women who are, or plan to become pregnant soon, should never consider the use of anabolic steroids. it would also be the best advice to stay away from these drugs completely for a number of months prior to attempting the conception of a child, so as to ensure the mother has a normal hormonal chemistry. Although anabolic/androgenic steroids can reduce sperm count and male fertility, they are not linked to birth defects when taken by someone fathering a child.