Final answer:
Braxton-Hicks contractions typically start in the second trimester and become more frequent in the third trimester as the body prepares for labor. True labor contractions are driven by oxytocin and involve stages of cervical dilation, delivery of the fetus, and expulsion of the placenta.
Step-by-step explanation:
Braxton-Hicks contractions typically begin in the second trimester of pregnancy. These contractions are irregular and often painless tightenings of the uterus that are not part of true labor. As the pregnancy progresses into the third trimester, the contractions can become more noticeable and frequent as the body prepares for the upcoming labor and delivery. The hormone oxytocin is primarily responsible for the contractions during true labor, which begin with the cervical dilation stage, leading to the expulsion of the baby, and ends with the passing of the placenta.