Physiological changes during the first trimester of pregnancy include, increased tiredness and lethargy, changes to breast tissue, nausea, vomiting and morning sickness, sudden changes in mood, often constipated, increased frequency of urination, headaches, heartburn, weight change, food cravings, the production of the hormone relaxin creating an increase in ligament laxity, blood pressure starts to fall, and an increase in core body temperature.