What it does: Like progesterone, estrogen is secreted by the corpus luteum until the placenta takes over. This pregnancy hormone plays a key role in the development of the fetus, with several organs and other bodily systems in the fetus triggered into development by estrogen. Once you've reached the end of the first trimester, your body has higher levels of circulating estrogen, and then the levels plateau. The role of estrogen is super-important: It helps to stimulate hormone production in the fetus's adrenal gland, it stimulates growth of the adrenal gland, and it enhances the mother's uterus, enabling it to respond to oxytocin (another pregnancy hormone; see below).
The downside: Elevated estrogen levels may also prompt spider veins, nausea, increased appetite, and skin changes including changes in skin's pigment. Some women, however, are lucky enough to experience the upside of a pregnancy 'glow,' which is largely attributed to estrogen levels.