Estrogen is just a group of steroids. Estriol is the major estrogen that appears in pregnancy, as opposed to Estrone and Estradiol that make up the predominant estrogens in post and pre menopausal women respectively (keep in mind this is very simplified) and are given in hormone replacement therapy (generally as premarin for estrone and everything that isn't premarin for estradiol).
So it's probably not as simple as simply "increased estrogen", which is pretty vague.