Intro to Pregnancy 03:55

A 40-week pregnancy is broken up into 3 phases, known as trimesters, with the hormonal profile changing with each trimester. 

1st Trimester (0-12 weeks): Human Chorionic Gonadotropin rises significantly (and what is typically measured in pregnancy tests) along with Relaxin, a hormone that promotes tissue laxity and greater joint mobility. Estrogen and Progesterone also increase, albeit at a slower rate.

2nd Trimester (13-28 weeks) and 3rd Trimester (29-40 weeks). Estrogen and Progesterone increase significantly along with Prolactin which stimulates breast growth and milk production. Relaxin levels off and remains relatively constant for the remainder of the pregnancy.