During pregnancy, changes happen in the mother's body to make sugar more available to the fetus, Glantz said. One of these changes is that the placenta produces hormones that interfere with the action of insulin, a hormone that helps sugar (or glucose) get from the bloodstream into cells. This means that sugar in the mother's bloodstream is less able to get into her own cells, leading to a rise in blood sugar. Usually, the fetus takes in this extra sugar, so the net result is typically that a women's blood sugar declines, Glantz said. But in some women, the placenta may put out too much of the insulin-blocking hormones, leading to an increase in the mother's blood sugar. If blood sugar levels rise to an abnormally high level, this is considered gestational diabetes.