My guess would be that it died about midway through pregnancy, stayed there and gathered a collection of minerals. So she wasn't pregnant for 46 years, just carrying her dead foetus for 46 years which is kind of morbid.
wow this is incredible... so many strange things can happen in nature Thor's hypothesis sound reasonable, but wouldn't the fetus decompose when it died?
While rare, it is not unknown. WARNING: Gross and icky pictures, not work safe. http://www.obgyn.net/ENGLISH/PUBS/ARTICLES/Stone_Baby.htm http://path.upmc.edu/cases/case128/dx.html http://www.taiwanheadlines.gov.tw/20000106/20000106s5.html
Not if it was preserved inside the woman's womb. Also, the articles states that the unborn baby is fossilized. So there we go, an explanation